I have a Beige G3 with a Ziff upgrade from 233 to 366, It has 192 mb of ram and I have OS 9.2.2 with OS X 10.1.5.
My computer would run better in X with at least 256 mb of ram for iTunes 4.0.1, However OS 9.2.2 runs nice and quick and it uses only 26 mb of ram.(I keep getting unexpected quits while using the I.E. browser with iTunes as a helper app in OSX)
If you are going to install X then put lots of ram in the Beige or keep the system at a minimum
I've installed all the extras into OS X 10.1.5 as well, Like Quicktime Pro 6.3. iTunes 4.0.1, iMovie, iPhoto, etc.(I'll have to get more ram)