Not knowing what all you will have in your PC, I would say get a minimum of 400W.
Why? Well, now-a-days putting so much in like a large Hard Drive & more than one, powerful CPU, powerful CDRW's or DVD's with double layer stuff, and especially the high end PCIe or AGP video cards, some requiring an extra additional cord plug in to the power supply.
Depending on your $ budget, it is best not to get a really cheap power supply, as they usually will not maintain the proper power to run everything even one with 550W, (costing $20).
Course' I just recently bought & installed this 420W inexpensive one as a replacement since I soon will be adding a DVD and 6600GT AGP card to one of my computers. Well, what sold me over on getting this one is the 305 review testimonals. Easy installation, so far works well, and for the price, I'll take a chance on it handling anything I can throw at it.
http://www.newegg.com/Product/Product.asp?Item=N82E16817153006
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