I'll second the recommendation of the Mini-ITX route and using Linitx.com
My previous home media server was a N10000 board in a 1U case with a couple of drives hanging off it. Worked really well and despite also running my mail, a couple of websites and a upnp media server I never had performance issues. The only reason I upgraded was because I wanted RAID and the Mini-ITX machine didn't have the internal space or enough drive interfaces to do what I want.
On a side note I do have a PIII 866 box in a SFF case that was actually a gigabyte NAS appliance in a previous life and then became my first media server..it may or may not have the >136GB drive limitation I can't be sure, there is also a slight complication during installation in that it is headless in the true sense (no VGA connector or PS/2 ports) So you have to either figure out how to do an installation with a serial console or do a basic installation on another machine or as I did, plug in a PCI gfx card for installation and stick a keyboard on the internal USB header.
Free to a good or bad home, or it is probably going skipside in the new year
Of course the other option is a NSLU2 with debian installed and a beefy USB hard drive, but then we are talking two boxes, limited performance and a bit of clutter.