For a system using two GeForce GTX 680 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 750 Watt or greater power supply that has a combined +12 Volt continuous current rating of 53 Amps or greater and that has at least four 6-pin PCI Express supplementary power connectors.
The Corsair Professional Series HX750W (CMPSU-750HX), with its combined +12 Volt continuous current rating of 62 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with two GeForce GTX 680 in 2-way SLI mode.
I would say about the same thing as "ko888" did above. Also, 750 watts is kind of low with little room to grow. Personally, I NEVER skimp on power supply brand quality or room to grow a bit more. Yes, the 750 as mentioned will work, but I would go with the Corsair or Silverstone line in about the 1000 watt range for your system to use now and grow a bit too.