Depends on what you want to do. Please list your full system so we can calculate better. If you want to keep things future proof go, go with a 750 or 850 watt power supply. Personally I would want a 850 with what im guessing you are running.
Initial reports indicate that a GTX680 will pull about 17 amps by itself. So that is about 27 amps for the 2 largest power consumers ( the CPU and GPU). Figure another 100 watts for a "typical" system, and that is about 420 - 450 watts. Figure another 180 watts for a second GTX680 and that puts you up around 630 watts.
If you know that you are never going to run SLI, a good 550 watt PSU (Corsair VX550, for example, will be "adequate". I estimate that a 550 watt PSU would run about 80% of capacity at load.
I would use a good 650 watt PSU with a single video card.
Likewise, a good 750 watt PSU would do for 2 cards, but I would use a good 850.