Hi again,
and thanks to Caarrie, though it didn't help. I don't know if my question was one of the reasons for the "read before posting..."-sticky. But anyway, I wanted to point out that before I asked about the graphics problem I in fact read the stickies regarding various issues but to me it didn't seem as if anything in there would solve my problems, especially as I do not run planeshift on an intel-Mac nor do I have an ATI-card installed (mine has an NVidia GeForce MX 4. I guess I should have pointed that out in the first place, sorry for that.
Nevertheless, I tried Caarries tip, and it didn't change anything. I probably should have been answering earlier but I had other, more important things to do, sorry about that. So the bottom line is that after the update of the graphics-drivers and after I applied the changes to the config-file my iMac G4 still froze twice, completely and I can't see any pattern in it, it just happened after some hours of playing. It's a bit annoying (the problem itself, I mean) and slightly frightening as I have no other app that freezes the whole computer and switching the Mac off manually always leaves a bit of a bad taste in my mouth, so to say.