If you're going to pour big money into your system, don't do it with junk for power. This is also a component you'll be able to take with you to your new PC. A Corsair 850W or higher is what I'd recommend for SLI. I say this because my system will reach to the 670W range when gaming with both 580s near 100% load. The Corsair 850W covers all the wattage you'll be using on the 12V and gives you a little cushion for whatever else you're doing:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139009
If you're going to OC everything, I'd go with the Corsair 950W or higher.
Once you get that under control, I'd suggest the 580s. SLI is great technology.
I've enjoyed my 2 580s. I can max out everything including anti-aliasing and get high framerates, though there is the occasional game that "drags" the framerate down into the 60s (ie Metro 2033).