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.
I'll check out Ubuntu. I've been trying to set up Debian on my older PowerBook G4, and I've had lots of configuration problems. This sounds like it's easier to set up on a Mac, and I'm glad that it still has the Debian package manager, really the only thing that drew me to Debian in the first place.
I'm still having trouble getting sound working, but that seems to be a common problem with all varieties of Linux. Xmms works just fine, but Gnome's sound player freezes.