One 8800GTX is way better than two 7950's slied.
It can be configured in SLI, though that has not been done, yet...
So if your thinkin about getting an 8800..get the 8800 and wait till sli compatibilty and get another one. No two cards (Including 7950's) from the 7k series will beat out a 8800gtx.. it seems like one 8800gtx has slight more power than a quad sli setup... plus BluRay compatibility for 8800gtx. Also 8800gtx is dx10 hardware... 7950 can play dx10 games but software for dx10 only, not full dx10 compatibiltiy.
The 8800gtx also alows full HDR and AntiAliasing both at the same time..the 7950's dont allow that...
8800 is more power efficient than two 7950's also.