Actually nevermind.
I've read around and it seems you need 700W minimum, not recommended but minimum with a recommended being about 800-850W if you want to overclock. A decent 800W PSU is almost double the price of a 550-650W PSU. And in two years there will probably be a GPU that will be around the same price as the added cost of a 800W+another 670 that will run on a 550-650W PSU given how important performance per watt is these days that will be arguably more powerful anyway. Especially with more demanding next-gen ports
Doesn't seem worth it to me.
Thanks anyway.
Where are you getting those figures from?
Take a look at this review http://www.guru3d.com/article/geforce-gtx-670-2-and-3way-sli-review/4
Measured power consumption one card
System in IDLE = 140 W
System Wattage with GPU in FULL Stress = 279 W
Difference (GPU load) = 139 W
Add average IDLE wattage ~10 W
Subjective obtained GPU power consumption = ~ 149 Watts
Measured power consumption two cards in SLI x2
System in IDLE = 160W
System Wattage with GPUs in FULL Stress = 433W
Difference (GPU load) = 273W
Add average IDLE wattage ~20W
Subjective obtained GPU power consumption = ~ 293 Watts
Measured power consumption three cards in SLI x3
System in IDLE = 170W
System Wattage with GPUs in FULL Stress = 583W
Difference (GPU load) = 403W
Add average IDLE wattage ~30W
Subjective obtained GPU power consumption = ~ 433 Watts
And that's with an old OC'd CPU and CCFL lights on it etc. This is what they recommend for a normal system
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.
A good 650w will be fine but as I said I'd personally go 700w just for the extra headroom if you were say to overclock.
You can get 850W for almost the same price. If you look at a game like Metro 2033 (which IMO has awful coding) it can be power consuming going up to 333W. I don't know if you would use 2 cards, that it would double the WATT. So 666.
That all depends on the TDP of the card.