pricewatch.com shops around for you. just type in what you want in the search, and it will bring up everything on the web for ya. its what i use when me and the roomie are buildin comps for people.
www.pricewatch.com has the cheapest of the good quality. and the cheapest of the cheap, depends on what ya want. but remember, it is always cheaper to just buy the parts and put it together yourself.