I've got the same model, and would tend to recommend 6.0.8. It works smoothly, and most of the apps and games I've wanted to run will work under it. If you need any applications that will only run under System 7, try any revision prior to System 7.1.1. After that and the processor and RAM requirements of System 7 really start to limit performance (though performance under 7 is pretty much always poky on those).