I used to run SLI GTX 660s a few years ago. It was a terrible experience. When they worked, it was great, but half of the time something would be wrong. Lots of games didn't have support ever, others only got support when Nvidia got around to putting out a driver update, and some had jitter problems. I would have been much better off buying a GTX 680 (I couldn't afford it at the time). The only way I'd recommend SLI to someone (for gaming at least) is if they are buying two of the best single-cards. That way, if SLI works, you get amazing performance, and if it doesn't, you still get very good performance.