I ran
Yellow Dog Linux on a Beige G3/266 tower.
It was certainly faster than Mac OS X on a QuickSilver 733.
Emmulation of course is slow, and I think thats an off topic discussion anyway.
To answer your question, loom001, you will have a fair amount of manual setting up to do. Its been my observation over the years that the more idiot-proof a disto appears to be, the more work you'll have to do to get what you really want done - say, changeing window managers.
From what I learned, I can tell you this, expect less in the multimedia area from linux. For one, there's no quick time. Big deal I suppose, but there's more. FireWire support isn't nearly where it is on MacOS. And I'm not sure if DVD playing is supported . Scanner support is low. As for audio, you'll probably have to tweak that a bit. But, most of this stuff is possable if you just RTM, and make use of resources like the various power pc linux mailing lists.
Also, you can do TONS with a nice program called
Mac on Linux, or MOL .
Running Linux on a Macintosh is great fun, and you will get a lot out of it, even if you don't choose to stick with it.