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.

http://homepage.mac.com/techedgeezine/2004-0129_pb1400-upgrade1.htm
Found this page, and it enticed me to get one myself actually. Just for the novelty I guess.
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[size:85]I own so many computers, it's unhealthy.[/color]