You could just use Disk Utility to set the drives in a RAID 1 array. Then they maintain a mirror image of each other. I am assuming you will be purchasing new drives so you can format them in this array when installing OS X.
Given that you are putting the drives in the computer the only reason to do a main drive and backup drive is if you want a backup in case of accidentally deleting a file. The main advantage of the RAID 1 is that you have an exact mirror image so any changes are reflected on both drives simultaneously.
Seriously though with that much data investment. I would do a RAID 1 array inside the G4 then perform monthly backups to an external drive. Which would then be stored in a fire resistant safe, or better yet. After all you could have redundancy across 50 hard drives and still be useless if the computer is stolen, destroyed by fire or other incident.
I really like having a central iTunes server as all computers can access the iTunes library and any computer can have its own playlists. By the way only allow one iTunes copy the privilege of organizing the library.
