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 (amps) on the 12 volt rail(s) can power a system with two video cards operating in dual mode. There are a few exceptions like the new ATI Radeon HD 5XXX series cards which use less power due to their energy efficiency.
Here are the ATI recommended power supply requirements for the new ATI Radeon HD 5770 and 5750 video cards that just became available:
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connector recommended
600 Watt and two 75 watt 6-pin connectors for ATI CrossFireX™ technology in dual mode
That's for the entire computer system.
Technical review: http://www.anandtech.com/video/showdoc.aspx?i=3658
Power consumption, temperature, and noise levels are very encouraging.