I run Linux on an iMac as well. My research pointed me to using YellowDog Linux rather than SuSE. Once you read the install instructions and do the pre-install planning, it works pretty well (including KDE, GNOME, music etc.) although I strongly suggest that if you're running OS 9 or OS X that you put them in and update them to whatever you need first. I've got it on a 400MHz iMac with 64 Mb of memory.
You should note that older models may need more work than newer models. The installation instructions (on the web site) point out gotchas for you.
Curiously enough, I also run OS X on this box, even though Apple recommend twice as much memory. It runs. Erm... let me see - it walks. Actually, it's not as bad as I expected. However, since this iMac will become my gateway box, it doesn't really matter (as the current gateway PC is on its last legs and keeps making strange noises). It's a shame that the people who make the USB ISDN adapter only provide i386 Linux drivers and not PPC ones. Still, I can use OS X to connect to the net and that works so that'll do me for now.
The overriding performance factor that I've seen with OS X is memory. On the 2 boxes that I've got with 128Mb, it's functionally acceptable but don't try to do too much at one or else it pages like mad. The box with 256Mb is a lot better in terms of performance and keeps the other half quite happy (moans and groans about MS Word docs aside - the sooner OpenOffice.org come up with a full build that he can use, the happier I'll be 'cos I can install that for him and then he'll be happy).
Rod (if you're running OS X, then fink is your friend)
