I'm gonna cast my vote for dual GTX 760's in SLI. That way you'll minimize any potential problems with SLI (blah blah blah lol), and you'll get great performance either way. Also, since you're considering a tri-SLI setup, I'm assuming you'd have room for a third GTX 760 down the road. Doing so would really put you ahead of the 660's. I'd say you'll need at least a 750w PSU, if not 800 or better; overclocking or not.