If you don't plan to go sli or a larger gpu, a solid 500w psu from Corsair (TX), Antec, XFX or Seasonic will be all you need. You might also want to look at the new EVGA line - great specs, solid construction and price effective.
About 500 minimum. I would say 600+ to be safe. Corsair, Antec, XFX, and Seasonic.