I would rather buy the 8800 if I were you ;)
better yet, an ATI 3750. I have the 8800 GTS 640MB, I'm disappointed with it. Basically I upgraded from a 7800 GTX, and didn't see much, if any FPS increase in any of the games I run, and only a very few games saw a graphic quality increase.
I haven't compared the 2 using FSX as I only recently got FSX and had already stopped swapping the cards out to verify I'm not crazy about how little difference the 8800 has made.
I have an Alienware machine that's only 2 1/2 years old, has the proper PCIe x16 slot, and does alright on most things.