i run osx on a g3 powerbook 333mhz. almost the same computer as your ibook. 66mhz bus and all.
i have used the computer for DP3, DP4, PB, 10.0, 10.1, and 10.2.
My machine has 512MB of ram and i think thats what makes it usable. Although I think 384MB would be ok as long as you stay away from RAM intensive apps. 256MB will NOT be enough though. I run photoshop on my powerbook regularly and find it fast enough.
i actually get impatient with the speed of os9 whenever i go back to it (i still need to go to os9 for viewing movies in full screen mode, gets choppy in osx).
OSX has a different kind of speed. it lets you do many things at once. while os9 does one thing quicker. So i often continue working while a file is being copied or a cd is being burned or any number of other tasks. Switching back and forth between apps is very fast and nice in osx. this is where os9 shows its age. sure windows will popup faster in os9 and your movies will play better, but osx will likely be a mroe productive platform due to the ability to multi-task.
Another thing to consider beyond speed is hard drive space. A standard install of jaguar takes 2GB. OSX needs free space on the hard drive for proper memory management in low ram situations such as you will encounter (try t keep at least 1GB free). So if your drive is small (4GB) you may not be able to use OSX.
lastly it is important in low RAM situations to reboot osx often (at least once every two days and as often as every few hours in high use situations). This is because osx will use hard drive space as ram and this slows down the system since hard drives are slower than ram. osx makes extensive use of ram so the longer you run the more ram you use. extensive use of hard drive to replace ram will slow down the system. rebooting is the only way to free up your ram.
