SLI would add nothing at that resolution, get a new monitor and keep saving for a 680.
You mean a 580?
SLI would make it so I get a solid 60 FPS with pretty much any game wouldn't it?
A 560 Ti will run the vast majority of new games at 60 FPS on a 1080p monitor. You have a better card and a lower res monitor. For the love of God don't SLI unless you get a 2560x1440 monitor or a dual monitor setup or something insane like that.