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.
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.
Corsair, PC Power & Cooling, and Seasonic are some of the 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. Lately we've been seeing a few other brands offering some high quality units. An example would be the Antec Earthwatts series which is an improvement over Antec’s older series like the low budget Basiq models.
Yes, I have numerous posts saved in MS Office. Spend enough time here and you keep seeing the same questions over and over and over. It made more sense to come up with the general rule of thumb and simply paste it in. Beats the heck out of typing the same answer over and over. I also have over 50 references bookmarked for quick access.
When ATI introduced the 4770 last Spring, it was supposed to be a very good general purpose, mainstream video card that was energy efficient. It just happened that games could be played fairly well when two of the cards were used in Crossfire mode. It is by no means a top of the line, high end video card.
If you are thinking about enhancing your gaming experience, then you might want to take a look at the ATI HD Radeon 5000 series. There are a variety of models and most are energy efficient. The exception is the 5970 which has two gpu's on one card. Currently it's the fastest card available, sucks up quite a bit of power, and costs big bucks.