Here is Guru3D's power supply recommendation:
GeForce GTX 670 - On your average system the card requires you to have a 500~550 Watt power supply unit.
GeForce GTX 670 2x SLI - On your average system the cards require you to have a 700 Watt power supply unit as minimum.
GeForce GTX 670 3x SLI - On your average system the cards require you to have a 850 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. nVidia itself recommend 500W for a single 670.Since your going to overclock CPU + GPU Add OC to that.http://www.nvidia.in/object/geforce-gtx-670-in.html#pdpContent=2
I would recommend Rosewill CAPSTONE Series CAPSTONE-550 550W Continuous @ 50°C, 80 PLUS GOLD Certified, Single +12V Rail, ATX12V v2.31 / EPS12V v2.92, SLI Ready, CrossFire Ready, Active - PFC Power Supply http://www.newegg.com/Product/Product.aspx?Item=N82E16817182068#top
Superflower made - 80Plus GOLD - Excellent quality - This is simply top of the line at a modest price . $75 and 5 yr warranty