As DavidY just said, it depends on the quality of the "500W" PSU. If it is truly capable of 500W (e.g. a Seasonic, Antec, or Corsair), you could run a HD7950 or GTX570 on it. If it is a CM, Logisys, Apevia, or Diablotek, the label is a lie, and it is probably capable of no more than 430W (CM) or 300W (the others). On the CM, you'd be able to run a HD7770. On the others, nothing more than a HD7750.