This is a sweet page:

http://www.extreme.outervision.com/p...ulatorlite.jsp

pick your processor and put your components in. it'll tell you what size powersupply you'll need.

most of the time you'll find you're using roughly 350 watts, but it depends a lot on your video card. you'll want to get a psu that has more than your max though, because while the computer will run you'll be pushing your psu a lot and it will have a greater chance of dying and supplying unstable power to your parts.