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 5000 series cards which use less power due to their energy efficiency. On the other hand the top of the line 5970 video card and the new Nvidia Fermi use more power.
A high quality 500 to 550 watt psu will have a +12 volt rail rated at 40 amps. A high quality 700 to 750 watt psu will have a +12 volt rail rated at 60 amps.
In addition the power supply should be at least 80+ Bronze certified for energy efficiency. There are some models available which have achieved 80+ Silver and 80+ Gold Certifications.
Before purchasing a new psu you will need to decide whether you will eventually have a pc with one or two video cards.
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.
Lately we've been seeing a few other brands offering some high quality units. One example would be the Antec Earthwatts and the Antec TruePower New series which are major improvements over Antec’s older psu’s like the Basiq models.