If I can add to mende1's post .... you probably know that OS X 10.5 Leopard is the last operating system that will run on a PowerPC. Operating systems after that require an Intel processor. If you happen to have an iPhone 5 and want to sync it with your Powerbook, you can't, as that requires iTunes 10.7 which in turn requires OS X 10.6 which in turn requires an Intel processor.
If you buy a used copy of OS X 10.5, don't buy a gray-faced Apple disk for another system, as those disks are locked to the system types they are delivered with, and OS X 10.5 Leopard was never delivered with the PowerBook.
New OS X 10.5 Leopard can be also bought from hardcoremac.com.
Powerbook G4s needed a minimum of 867 Mhz (1 Ghz is higher) to get to 10.5 or later without special add-on software called Leopardassist.
10.5 also limits your access to Classic as is explained in this tip:
Upgrade only if you really need to, as explained here: