Consider looking for older virtual synths and earlier versions of GarageBand, as well as other recording and editing software. Also consider setting up a Mac OS 9 boot environment. It has a much lower CPU overhead that OS X, and it will allow you to run even older synths and FX.

If you feel like tweaking, you could also consider Linux for PPC. There is quite a range of mature music software now. Again, you would want to aim for a fairly minimal OS and GUI, as well as finding older versions of software if possible