I'm purchasing a 750w psu. I've been leaning towards thaa option as well Ubercake, just for sheer upgradability.
A 750W is cutting it close if you'll be considering 580s in SLI. My power requirement for SLI when my GPUs hit in the high 90%s load while gaming is around 660W-670W (appliance load tester is reading around the 780W mark pulling from the wall). If you OC anything, your requirement could add to that requirement. I'd suggest you shoot for an 850W or higher (Corsair-like specs) if you plan to go with 2 580s to be on the safe side:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139009
It's not modular, but will give you what you need. I have the 950w edition. It's solid. I tried to get near 1000W based on advice from so-called "experts" I've seen on these forums, but when I got scientific about it found you need far less for 2 580s (as noted above).
A 650w can give you what you need for any single GPU card. Even a single 580, though a second will push beyond 650W.