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's not something you can add memory to, for example, as it involves the base architecture.