Sli GTX560 DCII OC issue....

As for as picture quality there's no difference, the only reasons people SLI for are to use more monitors or increase framerate in very demanding GPU intensive games. Right now there's a handful of games that work better with SLI and a lot more than dont. For example BF3 and crysis would utilize SLI to the fullest for example I know Assassin's creed 3 doesn't support it very well. An therefore people are getting worse frame rate with SLi on then just using 1 card. It all depends on if that gaming developer made the game to utilize it. And if they didn't you won't see an improvement in performance at all. In fact it will be worse than using one card. Now in the NVIDIA control panel you have the option to control the games, meaning you can choose if you want the game to use SLI or not. So it's just a trial an error process to see what games you play work better and which ones don't. If your using 3+ monitor at full HD then I would say a SLI upgrade would be worth it. But in my opinion I think your better off spending the money on a more powerful single card such as the 670 or above or even a 660-660 Ti which is 40-50% faster than the 560. As far as other hardware upgrades since your only using 1 monitor and not doing much heavy gaming then a SLI upgrade won't really help you very much.