I have actually tried both. Just swaped out my GTX660 SLI set up and installed a single GTX 680. While the FPS were higher in SOME games, the single 680 delivers a smoother gaming experience. Yes on some games the 660 SLI set up is just AMAZING and others there was almost no difference when just running a single 660 even on games thet were SLI supported. I just picked up my ASUS 680 from Frys Electronics, only $340 after rebate. That is $20 less than the two ASUS 660's it replaced. Of course now that I know how well SLI works on some games I'm thinking of a 680 SLI set up. Guess its time to start saving again.