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 power a system with any single video card made. A high quality 700 to 750 watt power supply with sufficient current on the +12 volt rail(s) can easily power a system with two video cards operating in dual mode. There are some exceptions such as the new ATI Radeon HD 5XXX series of energy efficient cards which require a little less power.
Corsair, PC Power & Cooling, and Seasonic are some of the brands with a reputation for high quality power supplies. They consistently earn high marks in technical reviews. They are reliable, stables and come with at least a 5 year warranty. Some Corsair models come with a 7 year warranty. Lately we've been seeing other brands offering a few high quality units.
The Corsair TX750 would be a good choice for a system with two ATI Radeon HD 5770 video cards operating in Crossfire mode. It's on sale at newegg.com for $109.99 and shipping is free:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139006