Currently Being ModeratedApr 10, 2013 3:18 PM (in response to DjFacts)
Choose About this Mac from the Apple menu and check the processor.
If it's a PowerPC G4, buy a retail Mac OS X 10.5 DVD from a source such as Amazon or eBay; Apple still might have copies in stock if you phone them.
If it's an Intel Mac, buy a Mac OS X 10.6 DVD from the online Apple Store.
Currently Being ModeratedApr 14, 2013 6:55 PM (in response to Niel)
After I buy and install Leopard in my Power G4 IMac, can I install Snow Leopard from the disk I purchased through the Apple Store for my Mac Book Air? What about the OSX Lion, or Mt Lion versions I bought and downloaded from the Apple Store for my IMAC Snow Leopard last year. I know there is a limit on what systems I can install but can't find which it is as of yet. Thanks since this is exactly the issue I too am dealing with today. the Snow Leopard disk I installed on my MacBookAir is the one that is, or was $20, that was mentioned above.