I have tried SLI 960s in the past, at 3440x1440 resolution. Verdict: unusable. I don't know if it was the lack of ram, but basically there were weird LONG periods of slowness when things got busy. You might get away with it at lower resolutions, or if both cards are 4GB models, but even then, you're better off looking for a single faster card. I just picked up a temporary use card: a new EVGA SC ACX 2.0 970 for £220 and they have £20 cashback on that bringing the effective cost to £200. Given a new 960 starts around £140, I think it is worth looking at the 970, or AMD 480, or give it a couple of weeks for the 1060.
If you go ahead with SLI 960's, they're not very power hungry and assuming you don't have an insane OC CPU or too many accessories, a decent quality 500W should be plenty.