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.
Before purchasing a new psu you will need to decide whether you will eventually have a pc with one or two video cards.
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. Lately we've been seeing a few other brands offering some high quality units.
If you are only going to install one video card, then the Corsair VX550 recommended by bige420 is an excellent choice. It is a very high quality power supply with a single large rail rated at 41 amps. Here is a link to a technical review:
Thanks JohnnyLucky, I had no ideas of eventually purchasing a second GPU and making an SLI system, until you've mentioned it Maybe I'll go for an 650W version of the same brand (corsair) for such a possibility. Would that be enough? Thanks for reminding me that.
And by the way, the technical review of VX550W looks great, pretty convincing. Unfortunately that PSU is not 80$ in my country. Its 125$+tax making it like 150$. But it is so a mandatory part of the PC so I'll have to pay I guess.
And thats just too expensive, I could go for a cheap brand, high watts but that would be unhealthy for the pc in the long run. Guess I'll stick to VX550 for now. As you might have noticed the other parts of my rig aren't too expensive anyway. It's a budget system. No need to try and turn it to a high-end for now. I was thinking even a GTS250 is too much for this system.