Mwave.com is also a good place. You would have two options. One, to pick out any components you want that they sell online (pretty much anything), and add $75 to build, test, etc.
The other way is to get a barebones setup, which is also assembled and tested...for free. Components you can have included are: case, psu, cpu, motherboard, memory, optical, fdd. You don't get access to their entire online stock, but you still get plenty of options, and pretty good ones at that. If you plan on buying an AM2 socket system, it isn't available yet, but "coming soon."