Are you planning on Crossfiring? How many SATA devices are you planning on running? Are you overclocking? All of these factors influence how much power you need, but other than the first one, don't really affect the PSU size too much.
I highly recommend sticking with Corsair, Antec Earthwatts or OCZ units. Without knowing about Crossfiring, I'd recommend the OCZ StealthXStream 700W. It's $80, with a $25 rebate, and is a high quality unit that will handle Crossfiring.
If you're not Crossfiring, you probably only need 500-600W. Just find a unit by those manufacturers, and you'll be fine.