Defining main rig is an interesting question - my systems are a bit inter-related!
1. IBM T30 with a faulty memory slot is probably the machine that I use as a computer most often.
2. Athlon 64 @?? GHz with 1GB
RAM - MythTV box
2. VIA Nehemiah 1GHz with 256MB
RAM - Firewall/mail server
3. PowerPC MPC8241 266MHz with 64MB
RAM - NAS server