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Mac OS X 10.4 (Tiger) needs to be purchased as it is not possible to update from Mac OS X 10.2.8
The 1.6 update requires Mac OS X 10.5.4. (See links below)
Mac OS X 10.4.11 is the final version of Mac OS X 10.4 (Tiger). To update further will require the purchase of 10.5 (Leopard) or 10.6 (Snow Leopard).
Mac OS X 10.3.9 is nearly five years old (which is a long time in computer operating system years) the current version of Mac OS X is 10.5.7. The minimum requirement for the latest version of the Safari browser is 10.4.11. To update your browser you will need to update your version of Mac OS X. Mac OS X 10.5 (Leopard) will work on an iBook G4.
To upgrade from Mac OS X 10.4 (Tiger) to 10.5 (Leopard) you will need to buy a boxed version of Mac OS X 10.5. If you have a Mac with an Intel processor you can go straight to Mac OS X 10.6 (Snow Leopard) which is a cheaper upgrade option.
Mac OS X is supplied with all Macs. Updates can be downloaded with the Mac's Software Update. New versions can be purchased from Apple stores and other retailers. There are no other legitimate sources.
MAC OS X Leopard which it's current version is MAC OS 10.5.8, maybe 10.5.9 in the future if they ever come out with another update for Leopard.
You need to purchase each version (Panther/Tiger/Leopard) of Mac OS X.Mac OS X 10.3 (Panther) can be updated to 10.3.9.Mac OS X 10.4 (Tiger) can be updated to 10.4.11Mac OS X 10.5 (Leopard) can be updated to 10.5.8
Click on the Apple menu in the top left corner and select About This Mac. This will open a window which will give the version of Mac OS X you are running. From the Apple menu select Software Update... which will check if there are any updates available for the Mac.
It's possible that the Mac OS X version that is installed on your MacBook does not meet the minimum system requirements for iTunes 9. The minimum compatible version of Mac OS X is 10.4.11. If your MacBook has not been updated to at least that version, just run Software Update ( -> Software Update...) until you're running 10.4.11. To tell what version of Mac OS X you're running, go to  -> About This Mac.
Mac OS X Lion will be release some time in July of 2011. UPDATE: Mac OS X Lion will be released ONLY on the Mac App Store no DVD's on July 14th, 2011
The main OS for Mac can be found on a Mac when it is switched on. The current main OS is called Mac OS X.