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./quote]
The card can be overclocked to 20+%. Guru3D measured the Asus GCII TOP model at 181 watts under full load w/ a 13% factory OC....adding 10% to that would add say 20 watts for each card plus say another 45 watts for a OC'd CPU. If overclocking the CPU and GPU, I'd be comfy w/ a 850 watter.
For the overclock you are looking for , I'd be looking for a PSU that can deliver 0 - 100% load w/ less than 1 % voltage variation. There are very few of those around....check jonnyguru for 10.0 performance rated units.
Antec CP series
Corsair HX series (except for 1000 and 1050 watt models)
Seasooic X Series.
XFX Black Edition Series