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Two 7970 non-Ghz cards will consume 500 watts under load. Add in another 200 watts for your motherboard, cpu, and ram. I'd probably get at least an 800 watt psu w/ decent efficiency.
If you oc the cards and get Ghz versions, I could see you pulling 300w per card. So then, I'd recommend a 1kw...
lol
The sapphire vapor-x run cooler than other 7970 cards because of their better cooler. I'd still expect ~80C though in a normal case with a decent oc.