I have quite a few macs G3 G4 G5 and the latest intel offerings. I'm going to have to be the bad boy here and advise you to chuck it, better to upgrade sooner than later, the Power PC area has long finished and apple no longer support any PPC programs having removed Rosetta from OS X.
Even my G5 Power Mac, the quad core 2.5 Ghz with liquid cooling won't out run a late 2009 intel mac mini. I bet (correct me if I'm wrong) it struggles to play videos on youtube ?
If you really need to keep it, like the others said a bit more ram, and a reinstall of the OS that shipped with it is the best option. On a side note who's trying Mountain Lion ?