PLEASE don't put 9.1 on that Mac. I did that on my parents' Performa 6220CD and it was DOG slow even though I maxed it to 64 MB. I haven't tried 8.5/8.6 on it, but I don't imagine it being any better (but I could be wrong). Mac OS 8.1 would be the max I would put on it and still maintain some performance, but if you can live without the quasi-preemptive-multitasking or HFS+ support then either 7.5.5 (which is free) or 7.6.1 (which you'd have to pay for) would be your option.