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iTunes 9.2.1 - http://support.apple.com/kb/DL1056 is the latest iTunes for Mac OS X 10.4.11. 10.4.11 is the latest operating system for Firewire capable Macs with PowerPC G3, G4 800 Mhz dual processor or less. If you have one of those machines with an older version of 10.4 (10.4.1, and 10.4.2 are older),
Upgrading to 10.4.11 using this link for G3s and G4s first is necessary before updating your iTunes, and you should backup your data before anything is installed:
http://www.apple.com/support/downloads/macosx10411comboupdateppc.html
That same update applies for G4 867 Mhz, and 1 Ghz G4 and G5 Macs that have software that isn't compatible with Mac OS X 10.5. 10.5 disables Mac OS 9 Classic environment, except for those Macs with Mac OS 9 booting:
https://discussions.apple.com/docs/DOC-1757
If you have 10.4.4 or later on an Intel Mac, you may want to consider updating to Mac OS X 10.6.8 which has better iTunes support. Go to Apple menu -> About This Mac and see if you have an Intel Core or Xeon machine, and then read this tip if you do:
https://discussions.apple.com/docs/DOC-2455
Otherwise if something is not compatible with 10.5 or above, 10.4.11 for Intel is found here:
http://www.apple.com/support/downloads/macosx10411comboupdateintel.html
Lastly, Wikipedia has an excellent article on what iTunes matches with what version of iOS: