I have a p4 1.8a CPU, a 60GB notebook hard drive, and an extra stick of 512 MB PC-2700 memory (not so-dimm). I want to build a system as small as you can get around these items. I have a USB 2.0 enclosure that I will be using to hook up optical drives with, so a CD/DVD drive isn't needed. Will mostly be used as a file or game server.
Have fun!