Here are the official ATI recommended power supply requirements for the ATI Radeon HD 5770 and 5750 video cards:
450 watt or greater power supply with one 75 watt, 6-pin PCI Express® power connector recommended.
600 watt or greater power supply with two 75 watt, 6-pin PCI Express® connectors for ATI CrossFireX™ technology in dual 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. In addition, some of the newer models have earned a gold certification for energy efficiency.