each 5870 has about a 150watt max load. 5970 is two of those, plus your 5870 means 3. So atleast 450watts right there. plus the rest of your system, another 300-400 watts. I would go 850watt, but if you only plan on using the 5970, then a 750 should be fine. Really, the only thing that scales well is power consumption, the performance increase per card goes down the crapper after 2 cards(aka single 5970). I'd stick with only the 5970.
Here are the official power requirements for ATI Radeon™ HD 5970 video card.
ATI Radeon™ HD5970 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
650 Watt or greater power supply with one 75 watt, 6-pin and one 150 watt, 8-pin PCI Express® power connectors
850 Watt or greater power supply with two 75 watt 6-pin, and two 150 watt, 8-pin, PCI Express connectors for ATI CrossFireX™ technology in dual mode
The power supply recommendations are for an entire pc system.
Corsair and Seasonic are two of the brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty.