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.
PSU recommendations: http://www.newegg.com/Product/Product.aspx?Item=N82E16817207017
( 20% off w/ promo code XFXP20, ends 12/12 XFX PRO850W XXX Edition)