I have a G4 400MHz w/512MB RAM desktop operating on Panther & a G3 400Mhz FW Pismo w/384MB RAM laptop operating on Tiger. Both operate adequately. Both run slower than newer, faster Macs. More RAM will increase operating speed.
Some advantages of OS X are more modern browsers that work on most web pages, view the latest Flash videos & .wmv files. New apps such as iMovie, etc.
If you keep OS 9.2.2 & operate in "Classic" in addition to OS X, you will be able to use most of your current apps.
I have OS 9.2.2 & OS X on separate partitions, so I can boot in either OS.
Here's the OS X requirements.
OS 10. 4 Tiger requirements
http://www.apple.com/macosx/upgrade/requirements.html
OS 10.3 Panther requirements
http://docs.info.apple.com/article.html?artnum=106163
Cheers, Tom