Looking at a few benchmarks I think the 560 SLI will most likely have a bit more performance, but at the cost of using more power possibly more heat and no future option of upgrading. All that aside do you have the power requirements for SLI and what resolution will you be playing at?
Power requirements? I'll be using a I5 3570k, with this monitor. Hopefully at 1920x1200. I also have a 700w power supply. Should I just stick with the single 660, I was also thinking about the Radeon 7950?