Rummaging through the bargain basement of Ebay, there's some great deals to be had.
This server, for example, is £40, and gets you a 1U box with two AMD Opterons each with two cores, running at 2 ghz. 8 gb of memory, two gigabit ethernet ports. Downside is, the disk controller is ATA, which means you'll only get four 300 gb drives, although I have a little thing that lets me run a SATA drive on an ATA port, which seems to work fine.
I'd expect to pay £40 just for a 1U power supply!
I am *so* tempted. Trouble is, I don't currently have a use for it, and I do have a couple of dozen working servers not doing anything.