From what I can see the 8800 GTS consistently out performs the 7950 GX2. There are only a few instances where the 7950 GX2 out performs the 8800 GTS and that is at high resolutions. If you don't have a huge monitor then I would buy the 8800 GTS over the 7950 GX2. Heck if I had to buy a card right now I'd buy the 8800 GTS. It's ready for DX10 and it seems to perform well. But if my pockets were deep the GTX would be the way to go.
btw this really bums me out cuz I just bought the evga 7950 GX2 just over 3 months ago. Puts me outside the Step Up program and I'm out of luck. Looks like I'm waiting a while to upgrade. I do have to say If your bent on haveing the 7950 GX2 it's a great card. Performs well and (at this point) takes anything you throw at it without hicups.