The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current (amps) on the 12 volt rail(s) can easily handle a system with any single video card made.
Corsair, PC Power & Cooling, and Seasonic are brands with a reputation for high quality psu's. They consistently earn high marks in Technical reviews. They are stable, reliable, and come with a 5 year warranty. The Antec Earthwatts EA-500 mentioned by gkay09 is also a very good power supply.