HT1338: OS X: Updating OS X and Mac App Store appsLearn about OS X: Updating OS X and Mac App Store apps
Currently Being ModeratedFeb 11, 2013 7:56 AM (in response to SuperBKV)
Your PowerBook G4 with its PowerPC processor can only be upgraded to Leopard 10.5. Since Apple has stopped selling the DVD for 10.5 the price for them is going sky high.
Be careful there are cheap ones but they are usually hardware specify and will only work with the hardware they were issued with.