Check the Dell outlet for refurbs..... there is a link on their page. Know kinda what you want, do the search query and then grab 4 or 5 and put them in your cart. This will allow you to look more indepth without someone buying out from under you. It'll give you about 20 minutes to hold them in your cart, so be ready with debit or credit card. I did this and got my desktop complete for about 249.00 shipped, but it did not include a monitor (didn't want one).
Another thing is to try the local computer shops, they may have something used, but I caution against the used ones as others have said, they do wear out.