The official recommendation for the Nvidia GTX480 video card is a 600 watt or greater power supply and an 850 watt or greater power supply for two cards in SLI mode. The recommendations are for an entire pc system.
Corsair and Seasonic are two brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty.