The lowest wattage PSU certified by nVidia for twin 580s is a 900 watter.
http://www.slizone.com/object/slizone_build_psu.html
Remember, it's not about turning it on and not having it crap out. It's about providing stable voltages under load for both overclocking stability and long term reliability. I use 950 watters minimum for twin 580's, 850 min for twin 570's and 750 for twin 560's though if pushing the latter to 1000 MHz, I'm more comfy w/ an 850.
Using the extreme PSU calculator w/ twin 580's, and 5.0 GHz CPU OC, 5 USB, 1 FW, 5 fans and card reader, and 15% capacitor aging, I get 886 watts
Using the extreme PSU calculator w/ twin 570's, and 5.0 GHz CPU OC, 5 USB, 1 FW, 5 fans and card reader, and 15% capacitor aging, I get 869 watts .... that number seems a bit high....I'd have expected maybe 785.
Using the extreme PSU calculator w/ twin 560's, and 5.0 GHz CPU OC, 5 USB, 1 FW, 5 fans and card reader, and 15% capacitor aging, I get 729 watts
And that's with
no overclocking on the GFX.
As for what to choose, if the target is SLI, it's hard to argue against twin 900 MHz 560 Ti's if the benchmark is frames per dollar. Their 862 fps score (48 cents per frame) in SLI is marginally behind the twin 570's which get 11 fps more at 873 fps (73 cents per frame) and surpasses the twin 6970's by 37 fps (825 fps @ 87 cents per frame) in Guru3D's testing. You can easily take the 560's further to 1000+ Mhz whereas the 570 has not shown that kind of headroom. The 580 is the undisputed champ but at 101 cents per frame in SLI it's abit of an investment.
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
$ 210.00 6950 (479/751) $ 0.44 - $ 0.56
$ 240.00 6950 Frozr OC (484/759) $ 0.50 - $ 0.63
$ 205.00 560 Ti (455/792) $ 0.45 - $ 0.52
$ 360.00 6970 (526/825) $ 0.68 - $ 0.87
$ 205.00 560 Ti - 900 Mhz (495/862) $ 0.41 - $ 0.48
$ 320.00 570 (524/873) $ 0.61 - $ 0.73
$ 480.00 580 (616/953) $ 0.78 - $ 1.01
The 570 has had some issues with its VRM ..... a web search will find you many stories of fried cards but you'll find most of these resulted from exceeding manufacturer's published voltage limits. Can't fault the manufacturers or the design for that except to say that enthusiasts are somewhat spoiled by "getting away with such tactics" many times in the past with other cards. The 560 and 580 VRM's do seem to be a bit more "robust" given the lack of instances with these cards.