GeForce GTX 680 - On your average system the card requires you to have a 550 Watt power supply unit.
GeForce GTX 680 SLI - On your average system the cards require you to have a 750 Watt power supply unit as minimum.
If you are going to overclock GPU or processor, then i do recommend you purchase something with some more stamina.
Thanks for the great info now when u said that i needed a PSU with more power/stamina. 850 will be a safe bet running 2 asus GTX680's DCU2 4G and over clocking them?
Also I will be over clocking the CPU (i7-3930) What I like about the one you mention is that it comes with a nice rebate =) but if I am in a better and safe position with something more than 850 watts well I will have to spend a little.