power book G4 10.4.11 upgrade to 10.5 or 10.6

A PowerBook G4 can't be upgraded past 10.5.8; check sources such as Amazon or eBay for retail Mac OS X 10.5 DVDs.

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The latest OS X version for PowerPC-based Macs is 10.5.8, so you can't install Mac OS X Snow Leopard.

Instead, check that your Mac can run Mac OS X Leopard > http://support.apple.com/kb/sp517 If so, you can buy Mac OS X Leopard at Amazon.

Then, make a backup of your Mac with Carbon Copy Cloner (there's a version for Tiger) and finally, insert the Leopard DVD and upgrade. After upgrading, open  > Software Update, and install the most recent Leopard version.

After upgrading to Mac OS X Leopard, you won't be able to use Classic applications