Openbox is nice, if a little dated by todays expectations of what a Window Manager should be. It's lightweight and based on the ideas of what Next Step should be as a spiritual successor.
It dates back to about the same time, when Next was still around and there were a few more clones, Openstep, Fluxbox, Afterstep, FVWM, Enlightenment etc... Some of them are around, some of the projects got dissolved. My first Linux install was Redhat 6.0 with Afterstep on PC many years ago now.
If you want to get a feel for what Nextstep might have been like if it were still around try Openbox, or Fluxbox. The good thing about both of them is that they're modular so you can install whatever dock, or menu, or anything else that you would like into it and have it look however you want.
For something lightweight that is OS X like you can try Openbox with Cairo Dock 2D as a dock replacement and Gnome Panel as an Apple Menu replacement that crosses between the OS9 and OS X style Apple menu, or just Gnome panel if you want something more like OS 9. That shouldn't take up to many resources.
The biggest issue I've ever found with Linux on PPC is video card driver support, no, software support or lack there of is not so much the issue. For older video cards there is a real lack of decent video card drivers. The open source drivers really don't provide the hardware support one would expect which makes it a hard platform to use for anything other than software necessity in 2012.
For basic tasks you will still do OK though.
