Well, currently i have 6 computers running at home...
Gaming machine - play games on and generally net surfing
Fileserver - running on Server 2003 64bit contains my RAID controller for all my storage (3.18TB, ISOs, music, tv, movies etc.), also serves as a torrent downloader, WSUS box
Domain Controller - Running on Server 2003 32bit, runs DNS, DHCP, Exchange and DC.
SCCM box - Running Server 2008 32bit, running my SCCM environment (System Centre Configuration Manager) and will soon be used for VM's
The other 2 are simple HTPC's one for lougne and other for bedroom.
I know this is a fair bit and most will never use that kind of thing. I use this environment at the moment to train/self learn to hopefully pass some Microsoft certifications to get a better job (more money). If i wasnt doing that i would probably get away with 3 machines and just have a HTPC, Gaming and the fileserver. If you dont have to store alot of files and your not interested in learning environments its probably not worth having a server at all for personal use.
EDIT: Unless you have a dedicated task to do in everyday life your more than likely wont need anything.