Hi all,
I'm going to archive my experience building a new PC with FSX in mind.

This is intended as a running archive - I would like anyone here to to what I did, and am willing to share knowledge and experience to anyone who asks!

So I had a dell p4 3.6 ghz machine, 2gb ram, GF6600 w/256mb which was fine (great for FS9) but did not like FSX much.  I wanted to update, and decided I could not exceed $500.  I would scavenge any non-obsolete parts I could from my dell (keyboard, mouse, monitor, sound card, optical drives) and purchase new everything else.  Given budget constraints, new technology was out, so second tier processors such as the E8500 core 2 duo, AMD phenom X4 9950 (quad core), and Q6600 core 2 quad were considered:

What the status?
It is up and running!

Quick EDIT:
All items were purchased from http://www.newegg.com - if you have the patience to wait for deals, you can get excellent prices from them and some considerable mail in rebates as well.  Links added to items below.  After I get the various rebates back, cost of upgrade will be $480.

Summary of parts:
1.  Rosewill ATX mid-tower case
2.  OCZ 600W power supply
3.  Intel Core 2 Quad Q6600 G0 (Stock 2.4GHZ, four cores) OEM
4.  ASUS P5Q SE PLUS motherboard (ATX)
5.  4gb (2 x 2gb) PC1066 DDR2 RAM
6.  Western Digital 3.0gb/sec SATA 500gb HD OEM
7.  Arctic Freezer 7 Pro CPU fan (stock fan not included in OEM, and not sufficient for over clocking)
8.  ATI sapphire 3850 512mb graphics PCIe
9.  Kept optical drives from my old dell

Some notes on these decisions (my logic):
The case:
Case only needs to have good directional airflow.  A place for a front chassis fan and a rear fan, preferably 120mm (large size) fans is great.  Cheaper is better.  Cases with built in power supplies tend to have low-quality power supplies.  I elected to go with the OCZ as it had good reviews at newegg and 600W should be sufficient for a single video card system.

CPU and mainboard:
My top priority was to get FSX running well on a $400-$500 budget.  There is quite a lot of controversy regarding what is best for FSX - the extra cores (duo v. quad) or raw clock (ghz).  I was compelled by the argument that games like FSX rely more on CPU computing power (cores) than speed, particularly a well-spoken fellow on tom's hardware who claims to have built many, and that FSX FPS improved by a wide margin using a quad-core CPU.  Additionally, the price point was a key - the Q6600 is at the top of the $150-$175 cpu range in terms of performance.  Equal in price to the E8500 (dual) and the AMD Phenom X4 (quad).  My decision to go with the q6600 was for the four cores, and that it reportedly overclocks much better than the AMD and better than newer intel chips which have a miniaturized architechture (45nm v. 65nm for the Q6600).  This means it overclocks better than the more powerful and newer intel chips.  For motherboards, I looked at the top three (MSI, ASUS, gigabyte).  ASUS had the only intel-p45 based board under $100, and their BIOS is second to none.  The only compromise I had to make was getting a single PCI-express slot.  This means I can never buy a second video card and link them in parallel.  Which, to me, is fine.  Any performance increase that two boards would bring could also be realized by a simple upgrade to a newer single.  If I ever have enough money to actually consider buying two video cards I won't be worrying about getting a new motherboard either.

Cooling:
The arctic freezer pro 7 is a cheap, effective air-cooled chiller.  I'm really pleased, and it was only $25.  I also bought a tube of arctic silver 5 thermal transfer compound for $6.  This stuff is apparently the gold standard for silver emulsions (pun there, read it again ).

Other stuff:
Pretty much went low budget on the rest.  RAM is dirt cheap right now, so I picked the fastest 2x2gb PC1066 I could find - the G.Skill was $60 with 5.5.5.15 timing (no idea what that means, but its faster than 6.6.6.18, and more affordable than 4.4.4.12).  Same for the HD.  I also had the ATI card from a recent purchase, so I kept it for now.  That card is probably the biggest bottleneck on my system right now, but I got it for $75.  An upgrade to the 4870 would probably be a huge improvement, but that's for a later date.


Assembly:
1.  Installed the power supply into the case with a couple of screws
2.  Installed the CPU into the motherboard by releasing a lever and dropping the chip into the idiot-proof socket, close the enclosure and snap the lever shut.
3.  Installed the thingy that covers the IO ports into the back of the case.  Each MB has a unique port layout, and most cases have a punchout to accommodate.
4.  I installed the motherboard into the case at this point.  In hindsight, it might have been easier to install the CPU cooling fan before, but I needed to make sure it all fit properly.
5.  Applied a 1mm x 3cm bead of arctic silver 5 thermal compound to the chip in the right orientation relative to the position of the four cores (simple instructions on website).
6.  Removed pre-applied thermal compound from copper heat sink on the cooling fan with a coffee filter (lint free) and some isopropanol.
7.  Installed the CPU fan by snapping the socket 775 pins into the holes in the motherboard.  It was really simple.  So many people complained online about this step, I was worried.  Clearance between the case and the cooler was an issue, but it took me all of 30 seconds to confidently attach the unit.  The swiss instructions might be typically overcomplicated as well...
8.  Attached all the cables to the motherboard.  CPU1 was the hardest to reach, the others went fine.  Required a little reading of the MB manual to find USB and front panel connectors.
9.  Installed RAM into "yellow" (preferred) slots
10.  Installed hard disk and connected SATA cable
11. Installed optical drives and connected IDE cable
12. Connect power to drives
13.  Closed the case and connects mouse/keyboard/monitor/ethernet/audio/power to the back
14.  FIRE IT UP!

First light:
1. First post required me to enter the BIOS.  Once I verified that the CPU ID was correct (Q660 and 2.4ghz), and that all the ram was detected and timing appeared correct, I accepted all defaults (Auto setting for most) and rebooted with my Windows XP pro SP2 CD in the tray.
2. Installed windows XP, installed visual studio 2008 (which in turn installs a lot of microsoft updates), then office 2007.
3. Installed antivirus software (AVG home edition free - http://free.avg.com/)
4. Configured my internet access and hopped onto microsoft update for the litany of patches.
5. Installed Crysis (game) and FSX (and FSX SP1 and SP2) and tested.

More about the FSX tests shortly...but I'll throw the overclocking info here:
Overclocking:
1.  Enter BIOS
2.  Set AI smart to "manual" to allow overclocks
3.  Upped the Q6600 front side bus (FSB) from 266mhz to 333mhz.  This gives a conservative 3.0ghz final clock.
4.  Leave all voltage and other settings on auto
5.  Reboot
6.  Posted without a hitch
7.  Ran CPU-Z to measure CPU performance - clock reported correctly as 3.0ghz (default was 2.4ghz).
8.  Ran CoreTemp to measure temperatures of the CPU.  Here's another area where some personal discretion is advised.  Some sites on the web claim the max safe operating temp of a Q6600 65nm chip is 70 oC.  Some claim sustained operation above 60 is not advised.  I have not a clue.  Intel of course remains silent.  I have decided to be conservative, and not push this chip beyond 60 oC.  Running at 3.0ghz, the chip idles at ~38 oC.  Barely a fever.  Thus, 3.0ghz is my base of operations.  I have not tried to clock further as of now, but i will   Under stress (prime95 testing all 4 cores at 100%) it runs 45-47 oC.  Thus, for a few edits to the BIOS, I'm getting 125% performance out of this chip, with hopes of pushing 140%.

Applications (read: games):
FSX was tested pre/post overclock and pre/post SP1+SP2 patching.  SP1 added support for more than one core (rediculous that was in a patch), so performace out of the box was not expected to be great.
1.  PRE-OC, PRE-SP
  A.  Loaded up my standard config: C172 G1000 @ KLEB with real weather.
  B.  My settings for FSX were 1440 x 900 full screen, scenery sliders at very dense (scenery complexity) and dense (autogen complexity), 50% traffic commercial and GA, road traffic 17% (I really dislike more), boats/ferries 20%, 100% mesh, 10m resolution, 60 cm textures, most else maxed.  Oh - water effect 2.med (this is complex reflection, but not all reflections are drawn)
  C.  My old PC could not run with these settings.  I could get it to hold similar at 15 FPS but I would have to turn off AI traffic, and reduce texture resolution to 1m.  With AI at 50%, new box holds 21FPS no problem.  If I set to unlimited, I get between 40 and 120FPS!!!  *HOWEVER* all of this tanks once I take off.  Not sure why, but FSX uses way more CPU power to actually fly than previous versions of the sim.  As soon as I apply full throttle, FPS drops into the 20's.  I can still hold a 21FPS lock, but not by much.  Great thing is that textures are SHARP.
  D.  Loaded up a problem area: Lear, KTEB (Teterboro, NJ, just west of manhattan).  Even new box is a slide show cruising down the hudson.  As soon as KLGA, KJFK, KEWR are in view, FPS drops to 8-14FPS.  Playable, but ugly.  Reducing AI traffic restores FPS to acceptable, but I like to see the competition!
2.  Post-OC, PRE-SP
  A. didn't test extensively, but FPS improved by a solid 20% in the KTEB test.  Minimum was 10 FPS, smoother, still maxed around 15FPS and could not hold the 21FPS lock.
3.  Post-OC, Post-SP1 and 2.
  A.  NEW WORLD!!!  KTEB test usually holds the 21 FPS lock.  The urban areas of NJ look excellent with all that autogen and stuff.  I can get into situations (wide field of view, both KJFK and KEWR in sight) where FPS dips to the low teens.  I still enjoy that, but some might not.  Also, the weather was reasonable (some clouds, nothing complex) - in heavy weather with a lot of clouds or layers, FPS might suffer.
  B.  All told, I actually REALLY enjoy FSX now.  It looks stunning.  Particularly outside of NYC (and probably other hubs like LA), its great.  I need to install FSG mesh, UTX and active sky advanced/X-graphics, but that should not affect FPS.  I will also install a traffic package to get rid of Soar airlines and bring in continental, etc.  That will likely affect FPS negatively, but I'll tweak and report back.

4.  Crysis.
  A.  I have never played that game before, but it's used as a benchmark since it really tests hardware hard.  See http://www.youtube.com/watch?v=hIRdbrvrAb4 for a video that just does not do it justice.
  B.  A copy came free with the video card, but my old PC couldn't run it.  It's a sci-fi "you and your high-tech suit vs. aliens" first-person shooter...generally not my cup of tea.
  C.  It is absolutely amazing.  The environment is simply stunning.  As close to real looking as I've ever seen in a game.  First time in a long time that I played something and really didn't think what they've done was possible on a PC.  Full foliage with shadows from each leaf...just superb.  It's like playing a movie.  Water shimmers, peripheral vision blurs, light changes with time of day...I could go on, but I'll just send some screenshots.

Next?:
1.  Overclock more:
  A.  I will try to boost the FSB a bit.  I should be able to get to 3.3 ghz given my low temps currently.  Reports on the net claim 3.4-3.6ghz with temps under 60.  The process is iterative: Tweak up FSB, tweak down voltage to minimize temp.  Eventually more power will be required than you can give the chip without cooking it, so it won't boot or will have errors during stress testing, and then one just reverts to last stable settings.  Nice thing about the ASUS Bios is that it lets you store a "profile" of settings, so if one fails, just load the last profile.  Also, in the next two weeks or so, the arctic silver should "cook in" and temps should lower 3-6 degrees.  Once that happens, I'll likely take on this project...
2.  Install Vista.
  A.  I have an academic edition of Vista 64-bit.  I reserved a 100gb partition to use to install Vista in a dual-boot config (choose OS upon startup).  I really want to see if Direct X 10 is worth the hype...The screenshots rarely do it justice: http://www.gamespot.com/features/6182140/index.html - my suspicion is that DX10 has those "intangibles" that bring atmosphere to an environment like lighting and fog.  Nice thing is I can try!

So, who's next?

Thanks very much for sharing your build with us Scott.  I hope you got to use your new machine during the recent ice storm.  When you come back online give us an update.  Stay warm my friend.

Ok Scott.  You have had the system running for a while.  Is there anything you would change in your component list?  Have hardware components change in the short time since your build that you would buy anything different?  Most of all thanks for such an in-depth post.  It will be really helpful for those of us planning on building a new machine.

Hi Scott,
Long time no post. I read with interest your new P.C build, i did the same late last year.
As you can imagine parts here are not easily come by.
However i managed to get a core 2 duo core e7200 processor 2.53 ghz, a Palit Nvidia 9500 gt 512 mb ddr3 video card and two gigs of ram.
plus a new mobo can't remember offhand the make.
i am waiting for 4 gigs of ram to come from south africa next week.
put it together and FS9 is off the clock frame wise and FSX has gone from 3fps to 24-25 on high custom settings on salt key but lower fps in urban areas, Iw ill have to reduce settings for those areas.
I must admit I have o/c the cpu to 3 ghz but I have been amazed at the difference of every game I play, yes I play Crysis at high detail and it just looks awesome.
All of the above set me back +- U.S.$450.oo which in our old currency would be many many zeroes not enogh space here to put them all down!!!
I will see how much difference the extra ram makes and will let you know.
Cheers and happy landings.

Hi
Well I got my four gigs of ram, installed and fsx at Salt Cay was 60+ frames, London City was 25 up from 3-7 if I was lucky.
That was using the settings at default which were quite high.
I need to experiment with the settings to see how high I can go.
Will let you know how it goes.
Cheers

Recent reformat.  Had set up dual boot XP/Vista, but decided to go exclusively Vista x64 as i found myself NEVER using XP.  I guess I got used to it!

Reinstall of FS9 and, of course, FSX - an essential reference: http://www.simforums.com/forums/forum_posts.asp?TID=29041

I've also been tweaking the overclock - I currently have a stable setup, running my Q6600 which has a native clock of 2.4ghz) at 3.2ghz.  There is a modest noticeable improvement over the 3.0ghz OC I previously ran at, particularly with the new vid card.

Anyhow, I'm happy to post more detail if anyone is interested.