Originally Posted by Qlitchford
Two 6800GT's are going to perform, in most cases, not quite as well as a single 7800GT.
6800GT's in SLi will perform better than a single 7800GT (assuming the drivers and everything are working as they should). however, a 7800GTX is about on par with a 6800GT SLi setup and a 7800GTX512 is better than a 6800GT SLi setup. theres also the issue that SLi may not be compatible with some games (even though that possibility is dwindling).
i pondered this same dilemma, and you see the route i took. however, if you do want to go SLi let me know, i have a 6800GT available for sale (good price, excellent condition)