This http://mac.linux.be/files/xorg/imac7.txt is the xorg.conf that mintppc9 users have with your machine. It should be the same as what I've given you. I've been assuming that those modelines that are listed shouldn't be used because they don't have the same name as those used in the screen section. If you want to check then download the file to your imac G3 with the wget command.