Ubuntu 9.10 runs fantastic on all the PCs I've loaded it on, but I've also used the Alternate Installation disc and not the Live CD they default you to. I've always had problems with that version when I used it years ago and have avoided it since.Gross. You could install 9.10 on it. It's not officially supported, so it's a bit harder to find.http://cdimage.ubuntu.com/ports/releases/9.10/release/
Watch out though, I've had terrible experience with 9.10 before, and apparently others have too, but I haven't run it on PowerPC.
Also, if you're very fluent in Linux, then you might want to look at the PPC port of Arch Linux. I've considered running this myself in place of Ubuntu, but you have to know what you're doing.
I have 9.10 running on my iMac G5 and it's fast, if not faster, than Tiger on the same system. The only issue you'll have with running Linux/ppc is the lack of Adobe Flash for websites. If you don't care about that, then you'll be fine. Incidentally, "Gordon" which implements a Flash player through Javascript, might make this problem a thing of the past. I hope that it actually takes off.
http://paulirish.com/work/gordon/demos/
Oh, and "DEATH TO MING!"
