gtx 680 Sli runs fine on a 750whttp://www.guru3d.com/article/geforce-gtx-680-sli-review/4
Here is Guru3D's power supply recommendation:
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 the GPU or processor, then we do recommend you purchase something with some more stamina.What would happen if your PSU can't cope with the load:
bad 3D performance
spontaneous reset or imminent shutdown of the PC
freezing during gameplay
PSU overload can cause it to break down