Or don't upgrade... Try Cabrio, its small and lite perfect for older computers and I believe that it still works on 7.6. Its not that bad, has some decent skins and it's easy to make playlist, nothing fancy though.
OS 9 would be too heavy of a load on that old Mac, plus you would have to find an even larger hard drive for not just your MP3s, but to carry OS 9. Remember those old SCSI HDs are not so easy to come by anymore.