A good 750W PSU (Seasonic, Corsair, Rosewill Capstone, etc...) would be fine for a PC with two 680s in SLI.
You shouldn't see any bottleneck.
Thank you for your reply! It's Corsair. I've heard some people going with 1000W PSU, maybe just to have the extra. But that's good to know that I could go with a 750W