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
Power consumption, temperature, and noise levels are very encouraging.
I checked several references but could not find a competent technical review of the Ultra 450 watt power supply. Then, I checked several technical reviews for other Ultra power supplies. The results were a mixed bag. The Ultra power supplies are not stellar performers but they are adequate.
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 psu’s.
I bought this system without the intent to upgrade much, besides perhaps a 2nd hard drive.
BTW. what exactly does the 12v rail power? Does it power the gpu and the processor? Anything else?
Also, I have read in other forums that having as many amps as possible is good but doesn't that seem excessive? Why have more power than the system will use going down one rail? Wouldn't this cause heat/efficiency issues?