Fleabay can bag you some bargains. I bought a Dell Poweredge 700 for the very same reason, 2x 36GB SCSI RAID1, 1GB RAM, 2.8ghz processor, DAT72 drive. It was 3 years old and I got it for £101.14!
Need to put some extra RAM and disks in it, but the server has 2 SATA ports so I'll bang in a...