Hi. I'm another old hand at FS. (back to version 1) Some things to consider. If you are going with an X4, go with the 965BE and OC or better, the new 970. Also a new x6, the 1075 to look at. FSX demands as fast as you can run. Even if you go with the x6, OC if you can. In any case, watch your temps. I have the 955BE and run a modest 3.6 OC over the 3.2 standard speed with a temp never exceeding 55C when running full out. That gives me a 5C cushion. Practically the 955 should not be run above 60C. Whatever AMD recommends for your processor. Consider the GA-890FXA-UD5. The 890G series has built in graphics and is a waste unless you want built in graphics. The FX series, e.g. 790FX and 890FX are the top chipsets. For the new generation, 890FX is the way to go for the X6 although they run the x4 fine. There are times you will think you are running Prime95 as your cpu is going to run flat out. Part of the FSX tradition.
FSX was designed some time ago and it does not do well with the current SOTA video cards. FSX much prefers nVidia cards as does FS9. SLI or Crossfire does little if anything. Don't laugh but the best cost effective cards are the nVidia 9800 GX2, the 8800 Ultra, upscale to 285 GTX and the ATI 5870. Not kidding. The new cards do things that FSX didn't dream about. One is DX10. I don't see much using it and it does have bugs. There is something in the 5870, NOT the 5850, that ATI did that does handle FSX ok. But you are talking $$$. BTW, run the video setting wide open. Don't mash it down to 30FPS, let it do what it can do.
Another is you don't need over 4GB of memory, i.e. 2GBx2 sticks with the tightest timing you can afford. DDR3-1600 sticks are fine, faster if your budget allows. Since the board runs dual channel unganged, use only 2 sticks. If you want more than 4GB (FSX won't use it) go with 4GBx2 sticks. Do not run over 2 sticks or you'll have to down clock the memory, due to issues with the CPU built-in memory management.
Running FSX means running on its own drive and that drive should not be over 50% to 65% full and defrag like there is no tomorrow. Best if you don't run it on the same drive as the OPSYS and put FSX as the first thing on the drive. If you can get a VelociRaptors so much the better. Use a small page file on the same disk as your OPSYS. Unless you are doing colossal processing, you will never use the page file.
As to running all the sliders full right. Forget it. FSX is the most sensitive balancing act you can imagine. There are plenty of good articles on the web on tuning. Part of that tuning is the OPSYS, in my case W7 x64. Drivers up to date, both service packs for FSX.
Bottom line. As many CPU cycles as you can give it on as many cores as you can give it and 4GB of RAM. Use an ATI if you have $$$ and shoot for the 5870. If you are going SLI, you'll need something like the ASUS M4N98TD EVO
Cheers
A great link if you are interested:
http://www.simforums.com/forums/topic29041.html