u can buy a amd 2900+ and a mobo for jsut $125 at newegg.com i wouldn't pay more then probably 350-450 for that computer a nice crt (got mine for 100) jsut have to look for deals.
http://www.newegg.com/Product/Product.asp?Item=N82E16813135205
http://www.newegg.com/Product/Product.asp?Item=N82E16813135205