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 GPU or processor, then we do recommend you purchase something with some more stamina. Source: http://www.guru3d.com/articles_pages/geforce_gtx_680_review,9.html