So 2 670s will work fine?
BTW my build is: http://pcpartpicker.com/p/8HNn
but this build only has 1 670
Power consumption then, two GPUs consume roughly 300 Watts
whilst three GPUs use roughly 450W, add to that the rest of your system (processor, chipset peripherals) at say 200 Watts
and you'll find yourself in the 650~700 Watt power consumption region
when playing a hefty game. If you plan to overclock CPU and GPUs, then make sure you add another 200W onto your PSU requirements.
Our recommendation really is to go with an 800~900 Watt power supply if you plan to go with 3 GeForce GTX 670 cards and a beefy system. Overclocking equals increased voltages equals increased power consumption equals much more load on the PSU.
though they are extremely understating the cards wattage draw and overstating the rest of the computers draw, the rest sounds about right. You can probably get away with an overclocked cpu and 2 gpus at stock, but my knowledge of power consumption past that point is probably very limited, as i usually work and think with budget builds.