You'll want to use a version of Ubuntu that still supports Power PC processors. As of 6.10, PPC is only supported unofficially.
Try: Ubuntu 10.10 Ports
The first download link on this list is Ubuntu 10.10 ported to Power PC. If you feel brave, you can try the "Minimal Install" closer to the bottom of the page. This doesn't come with Gnome, and this way you save the space by only installing the Desktop Environment of your choice, but requires some comfort with the command line and installing Linux without a graphical installer.
Gnome (the default desktop environment) might be a bit demanding for that computer. If you find it to be very slow, I would try installing LXDE instead. It's a nice Desktop Environment that uses even less resources than Xfce in Xubuntu.
There is a guide to install LXDE here: LXDE on Ubuntu 10.04 (Don't worry about the fact that it says it is for 10.04, the instructions will work the same on 10.10)
Simply type "sudo apt-get install lxde" in a terminal once Ubuntu has been installed, restart your computer or log out, and select "LXDE" from the Sessions Menu on the bottom of the log-in screen.