it depends benchmark wise the 6600's beat the 6800, but gaming wise the 6800 beats the 6600, mainly becasue there are only a few if any games that take advantage of SLI
aj is correct....technically the two 6600GTs would perform slightly better, but not a huge difference really...you'd be better off buying a 6800GT and OC'ing to Ultra specs and perhaps later purchasing another......especially after the G70 is released the prices will drop