This evening I checked out Pricewatch.com to see what a mini-ITX motherboard would cost, and found this one for the stunning price of $40.
So I looked at cases and found a decent case with a 250W power supply and two 3.5" drive bays and such for $51.
Memory: $23 for one 2GB DIMM.
Everything else I already have on hand: I'll use the spare 500 GB drive as its hard drive, and as the case also has a 5.25" bay I can throw in my spare DVD +/-R/RW/CD-R/RW/WTF drive. I still have the copy of Vista I bought back in 2007 when I was going up upgrade my P3 to a Pentium D, so I've already got a legal OS to install. And I've got enough keyboards and mice and speakers laying around that I could probably host a LAN party.
...I just bought a new computer for $116.
The mobo uses an Intel Atom processor running at 1.6 GHz and it draws all of 8 watts as it does. With 2 GB of RAM it will not be a powerhouse, but then I'm not going to be playing WoW on this thing; all I'm going to do with it is download torrents.
It has one full-size PCI expansion slot. So, if I were to (later) buy a bigger case with more drive bays, I could get an SATA controller that could handle a bunch of drives, and install a RAID array in the thing. Install FreeNAS and presto! Instant NAS box!
...so I'm kind of excited. Yes, it's not a smart thing to do considering my situation right now, but I never claimed to be wise.