Hmm, im not sure now. There was only a 1FPS Increase over a stock 8800GTS on the X2900XT model, i think 8800GTS Superclocked of OC2 is the way to go, and it uses less power
If you decide on a 2900 dont get the XT you can just flash the bios and overclock it to be identical to an XT. I currently have the evga superclocked gts 320 and it works fine, but considering the resolutions i run its probably not going to handle crysis.
EVGA waaay better company. They cover OC'ing damage and they dont care if you put an aftermarket hsf on them as long as when you you RMA it (if you had to for some reason), it is in same physical condition. Great company, its worth the 5 euros or whatever