nVidia itself recommend 500W for a single 670.http://www.nvidia.in/object/geforce-gtx-670-in.html#pdpContent=2
For testing conducted for SLI 670, the resulting system consumption for SLI is 433W of which 293W are attributed to the gpu cards.http://www.guru3d.com/article/geforce-gtx-670-2-and-3way-sli-review/4
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. 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.
All points out to 750W being more than enough.I recommend this one http://us.ncix.com/products/?sku=62224&vpn=P1750BNLG9&manufacture=XFX&promoid=1257