Mac OS X 10.6 (Snow Leopard) will not work on a Mac with a G4 processor as it requires an Intel processor based Mac. A G4 (867 MHz and faster) can run Mac OS X 10.5 (Leopard).
The iMac G4 is able to run any version of Mac OS X up to and including, version 10.5 (Leopard).
You should be able to find a G4 iBook for under $500 with Mac OS X 10.5 (Leopard). Apple requires at least a G4 (867MHz or faster) processor with 512MB of memory to run Leopard - so anything above that will be fine.
Mac OS X 10.5 (Leopard) will run on a G4 providing it is running at 867 MHz or faster. At least 9 GB of hard disk space is required and 512MB of RAM.
To upgrade from Mac OS X 10.5 (Leopard) you need to buy a copy of Mac OS X 10.6 (Snow Leopard). Snow Leopard only works with Macs with an Intel processor; Macs with PPC processors (G4 or G5) can only go up to Leopard.
Mac OS X 10.6 (Snow Leopard) is only legally available to buy from a software retailer.
Yes. However, Mac OS X 10.5 (Leopard) requires an 867 Mhz G4 processor. Some of the older eMacs do not have a processor this fast, and so cannot run Leopard without third-party workarounds.
Mac OS X 10.4 (Tiger) can be updated to Mac OS X 10.4.11 for free. Purchased upgrades then are Mac OS X 10.5 (Leopard) or Mac OS X 10.6 (Snow Leopard). Snow Leopard only works on Macs with an Intel processor. Older Macs with a G4 or G5 processor can only go up to Leopard.
If your current version of Mac OS X is 10.4 (Tiger) or 10.5 (Leopard) then it will cost around $29 (£26) to buy the update to 10.6 (Snow Leopard). Snow Leopard only works on Macs with an Intel processor and will not work on older G4 and G5 Macs which can only upgrade as far as Leopard.
Although the appearance of Mac OS X 10.5 (Leopard) and Mac OS X 10.6 (Snow Leopard) is similar there are many differences in the workings of the two systems. Most notably Snow Leopard requires a Mac with an Intel processor whereas Leopard will run on earlier Macs with G4 or G5 Power PC processor as well as the newer Intel based Macs.
If you want to upgrade the OS the G4 can run Mac OS X up to version 10.5 (Leopard) which can be purchased from any Mac software supplier. To use 10.6 (Snow Leopard) will require a new Mac with an Intel processor.
Apple keeps an archive of manuals on their website. There are manuals for the A1013 - PowerBook G4 (See links below) but specific details will depend on which operating system it is running as it can handle all versions of the Mac OS up to Mac OS X 10.5 (Leopard).
To upgrade beyond Mac OS X 10.4 (Tiger) you will need to purchase Mac OS X 10.5 (Leopard). If the iMac has an Intel processor you can jump straight to Mac OS X 10.6 (Snow Leopard) but if it has the older G4 or G5 processor then 10.5 will be as far as it can go.