Ok, first = go to the new icon you want to use.
HighLight it (or actualy darken it), and go to the FILE menu at the top of your screen. Choose: GET INFO. (just in case) Then go to the next menu over (EDIT MENU.) Choose: COPY. Then go to the submunu in the applications folder called UTILITYS. Go to the "system prefrences" icon (or App, thats in the utilitys folder, and "Highlight:, or (darken) it. then go to the file menu at the top again, and GET INFO for that icon (just in case), and then go to thenext menu again EDIT, and chose paste. That's it., You wont see it change yet because it's a system icon,. Now if your system prefrences icon is in the dock, close it, or poof it out. Then Go to the Apple Munu at the top, and use Force quit on your finder,app. After you force quit your finder your new icon will be visible. and you can launch system prefrences, and your G4 tower icon will popup in your dock.
I hope that was not too much info.