The general rule of thumb is a high quality 500 to 500 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 4770, 5850, and 5870 cards which use a little less power due to their energy efficiency.
Before purchasing a new psu you will need to decide whether you will evdentually have a pc with one or two video cards.
I'm only going to use one.
I have a smaller screen atm so my resolution is 1280x1024.
My screen also uses a VGA adapter so will this slow down the GPU any bit(since I have to use DVI to VGA converter)?