Earlier this year I replaced my desktop PC, a Dell Dimension 4700, with a Mac Mini as part of my effort go 100% Mac for my home computing. OS X is a terrific platform for the desktop power user because it looks great, feels great, is well supported by open source projects.
Read it here
Since then, the Dell has been sitting by my desk, waiting for a good use. I already have a Linkstation Live that I use for a low power home server (print server, mt-daapd, subversion, samba, etc.) so I didnt have an immediate need for another PC around the house.