You have to understand the difference between an Update and an Upgrade.
An Update is a free change to add to the base code or as a bug fix within an existing Operating System. Such as the .8 added to 10.5.
An Upgrade is a complete new Operating System that must be purchased. Such as 10.5 Leopard to 10.6 Snow Leopard.
10.5 Leopard was based on Power PC code and was written to be able to run on both Power PC and Intel processors.
10.6 Snow Leopard has completely rewritten code and only runs on Intel Macs.
10.7 Lion used some 32 bit which allows it to run on Late 2006 through Early 2008 MacBooks and a lot of 64 bit code.
10.8 Mountain Lion is pure 64 bit, which excludes more systems than Lion did. It can only run on the Late 2008 through Mid 2010 MacBooks. It's not something you can add memory to, for example, as it involves the base architecture.
If you’re happy with 10.5 there’s no need for you to upgrade. But realize that you can’t update any applications. Companies like Adobe, Google, Firefox, etc.. no longer support anything less than 10.6.
The 10.6 Snow Leopard DVD is back in the Apple online store. You can get it for $19.99. You will need to be running 10.6.8 to access the App Store to order Lion or Mountain Lion. http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
