I used to have 2 x Gigabyte 6800GT 256MB cards in SLI mode with 3 GB RAM.
I bought 2 Nvidia 8800GT 512MB cards used from Ebay but my Mobo was in such bad shape I eventually rebuilt my PC with an ASUS M4N82 Mobo and AMD 11 X4 635 CPU , 4GB RAM and the 2 88ooGT's.
The new PC gave me hell, crashing all the time, until I eventually found that one of the cards appears to be faulty.With it removed the PC runs sweetly and gives very smooth running except in very high detail areas.
Moral; whatever you get don't buy used.