There are several you can try out and an entire website dedicated to linux on powerpc. See:
http://penguinppc.org/mac/
and:
http://penguinppc.org/about/distributions.php
I believe yours would be considered a "New World" G3 Mac, 32 bit. You may also want to take a look at debain's most recent release, Lenny. Debian has a long history of supporting all sorts of exotic architectures whereas ubuntu has been on again off again in its support of ppc. Fedora and Yellow Dog are probably worth a look as well.