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 .11 added to 10.4.
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 written for Power PC Macs with code added to allow it to run on Intel MacBooks 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.
The 10.6 Snow Leopard DVD is 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
System Requirements for 10.6:
- Mac computer with an Intel processor
- 1GB of memory
- 5GB of available disk space
- DVD drive for installation
Some features require a compatible internet service provider: fees may apply.
You can upgrade from 10.4 straight to 10.6 with no problems as long as you have at least 1gb of RAM. Any program that runs under 10.4 should run under10.6. See this list for third party programs compatibility with 10.6: http://snowleopard.wikidot.com/ You might have to upgrade some drivers for printers, etc.... And you will have to install Rosetta if you have any Power PC applications
I recently upgraded an older MacBook (Early 2006 model 1,1) straight from 10.4 to 10.6.
A retail 10.5 Leopard disk will run you over $200
10.6.3 can allow you to upgrade free to 10.6.8. Beyond that you have to be careful because compatibility with applications becomes hit and miss thanks to what this tip mentions: