But since leopard is based on and added to Darwin, wouldn't Darwin OS run better than leopard on a machine with only 512MB RAM? The reason I am really doing this is just to have a little fun with an iBook that I am no longer using at all since I moved to the 14".
Well, since you've moved on to a 14" it wouldn't hurt to try it out then. The PowerPC releases are still around on the Darwin 'site, I think.
I actually wanted to try this out but it looked like there wasn't much support on the older releases and the newer releases didn't support PowerPC.
Edit:
Check out PureDarwin, I think it has a version corresponding to 10.5.8 for download.
I was wrong about the support, it actually seems to have quite a bit. MacPorts, GNU-Darwin, Fink, Homebrew, RPM, pkgsrc and Portage - which I think are software-repositories - and then there's also separate applications you might be able to find too.
There's also PureDarwin nano which is supposed to be a more minimalistic release of PureDarwin.