I have 5 actually working, and another 4 in various states of undress !!
Main rig is a Barton 2500@3200 on my trusty old NF7-S. Server is buit around a 2.8 P4 on a Asus P4P800S-X. The 2 kids both have a budget P4 build each and my download box is a XP2100+ in a SFF case, nothing spectacular although I'm looking for a half size gfx card as the onboard vid is a little weak. (All running BOINC by the way).
Loads of other bits, all working but having difficulty finding the time to put them all together, I reckon another 4 machines could be built from the shambles that is my study. Plus her indoors is constantly giving me grief about the bits.