Look at these guys http://www.pcusa.com/ just remmeber you get no more than what you pay for. A friend of mine builds and resells PCs and these guys are local for him and usually have prices I can't beat.
Down side of the "low bidders":
Crap generic power supplies
Cases may have poor cooling flow, ask about case fans.
Watch return adn restock policies.
They usually substitue parts without telling you (this is sometimes an upgrade which is good).