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you could probably make due with a decent 500 watt psu with single 12v rail, but i would get a 650 watt for more headroom. but not all psu are created equal, just stating a very general idea here.
If you're only sticking with that GPU or something of a similar power rating, this PSU should be enough: $58 - Antec NEO ECO 520C 520W PSU
With that said, this is a fantastic PSU deal that I HIGHLY recommend that you get in on: $90 - Seasonic X650 Gold 650W Modular PSU
As HaX said, not all PSUs of the same wattage can actually provide the rated wattage without killing your hardware. So in other words, just because I recommended a 520W PSU above doesn't mean that ALL 520W PSUs will fit your needs just fine. Only those that can provide 520W without killing PC hardware will fit your needs just fine.