I've looked at Yellow Dog Linux and of course, the latest release (6.1) doesn't support this machine. However, I've tried earlier versions and wasn't too impressed.
I know there's also a community supported version of Ubuntu, and I've seen a beta of 9.04 for PPC, but this machine barely meets the RAM requirements (384MB minimum, which I have).
I'm actually taking a look at Fedora 10, which has low requirements and is a fairly recent release.
Thanks for the help though!