Well my first question would be what resolution does your monitor run at? If you’re running at 1280×1024, there’s no reason to get the 8800gtx instead of the the 8800gts.
Its been my experience that the 8800gtx runs ~25% faster on most games (on resolutions >= 1440×900 in games that are not cpu bound), at ~25% higher cost, making them basically linear on the price/performance curve.
Therefore, my suggestion would be to go for the 8800gts unless you are running resolutions of at least 1600×1200 or your cpu is something along the lines of the core 2 duo e6600 or higher, so that you are not running into a situation of your games being cpu bound.
Likewise, if you have a high end cpu and run at higher resolutions, I would suggest spending the extra for the 8800gtx as the difference should be noticeable especially on games like bioshock.
Just fyi a lot of the gaming club members I talk to have either the 8800gts or the 8800gtx so they are both really good cards and the brand preference I’ve noticed is eVGA (lifetime warranty ftw!). If you need benchmarks, I can get a couple for you but from my memory, MasterP and I ran a benchmark on CS:S:
everything maxed with 8xAA and 16xAF,
my computer: e6600, 2gb ram, 2x160gb raid0, evga 8800gts 640mb, @ 1680×1050, cs:s test gave my computer ~ 189 FPS
MasterP’s computer: q6600, 2gb ram, 2x250gb raid 0, evga 8800gtx, @1440×900, cs:s test gave his computer ~220 FPS
Hope this helps!