Lurk on gotapex.com or fatwallet.com in the Hot Deals forum and watch for good deals on Dell servers. I have three SC400 servers (P4 2.8ghz HT) which were all bought for less than $300 each after rebate. The SC400 is obsolite now, but the SC420 is a good cheap server and they often have good deals on them also. You can't go wrong with a Dell...
PS: I also have a referral link which can get you an extra 1.5% off purchases from Dell Small Business. PM if you want it.