nVidia itself recommend 500W for a single 670.You'll be finehttp://www.nvidia.in/object/geforce-gtx-670-in.html#pdpContent=2
Here is Guru3D's power supply recommendation:
Radeon HD 7950 - On your average system the card requires you to have a 500 Watt power supply unit.
Radeon HD 7950 Crossfire - On your average system the cards require you to have a 700 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.
System in IDLE = 165W
System Wattage with GPU in FULL Stress = 300W
Difference (GPU load) = 138W
Add average IDLE wattage ~3W
Subjective obtained GPU power consumption = ~ 138 Watts
Mind you that the system wattage is measured at the wall socket. Source: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&ved=0CFYQFjAG&url=http%3A%2F%2Fwww.guru3d.com%2Farticle%2Fhis-radeon-hd-7950-review%2F8&ei=op1KUNHWD8v6qAGt6IG4Cw&usg=AFQjCNGEjw0EzJ2bjJUvf5QlNquKej2_9g&sig2=SKMJjR_bAgZLMXdkZ2D_LQ