Last week I installed a 120G drive, which I got for $85 from CPU Solutions, in my G4 minitower, replacing the old 40G secondary drive (it also has an 80G drive). Since I have lots of disk space, I partitioned it for Linux as well as Mac OS X. I also have Jaguar installed on the smaller drive so I can test software with it.
Today I installed Ubuntu Linux on it. The installation was very fast & painless. I first partitioned the drive with Disk Utility, leaving a large unallocated space as well as the OS X partition. When I booted from the Ubuntu CD, it created the Linux partitions and did the rest. Less than an hour later, I was running Linux. Ubuntu is based on Debian, so I was able to easily add packages and update everything to the latest version using Synaptic.