Well.. technically those machines will max OS 9.1.
You can get the G3 upgrade and run classic.. and with a lot more RAM.. bigger HD and XPostFacto *maybe* OS X. I can't remember the terms of the PostFacto installation but I know it's been done.

Found this page, and it enticed me to get one myself actually. Just for the novelty I guess.
[size:85]I own so many computers, it's unhealthy.[/color]