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 2x SLI - On your average system the cards require you to have a 750 Watt power supply unit as minimum.
GeForce GTX 680 3x SLI - On your average system the cards require you to have a 900 Watt power supply unit as minimum.
Remember, if you are going to overclock the GPUs or processor, then we do recommend you purchase something with some more stamina. The minute you touch voltages on the CPU or GPUs, the power draw can rise real fast and extensively. SOURCE - http://www.guru3d.com/articles_pages/geforce_gtx_680_3_way_sli_review,4.html