you might look into one of the power sellers of barebones systems on E-bay. Once you win an auction...most allow you to add the components you want, and have the system put together for you....including an OS should you require it.
I had a pretty good P4 system with advanced Vid card put together for a friend for well under $500 awhile back (without OS).
Worth a look anyway.
http://cgi.ebay.com/ws/eBayISAPI.dll...139018580&rd=1
here is an example....please dont buy this one....just look and check the upgrade options