To upgrade from OS X 10.5.8, you must first buy Snow Leopard by calling the phone number in the Apple Online Store: 800-MY-APPLE (in the US)
Make sure your system meets Snow Leopard's requirements:
To install Snow Leopard for the first time, you must have a Mac with:
- An Intel processor
- An internal or external DVD drive, or DVD or CD Sharing
- At least 1 GB of RAM (additional memory is recommended)
- A built-in display or a display connected to an Apple-supplied video card supported by your computer
- At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools
The iMacs in this forum cannot run a Mac OS higher than OS 10.5.8, so its vitally important to at least find your processor type from "About this Mac" under your Apple menu before buying anything.
About this Mac" will give you this window:
If the processor information includes the term "PowerPC" as in the example. you are stuck at 10.5.8. If it says
"Intel" you can install 10.6 or even higher, but we'll need to ask the Hosts to move you to the forum for Intel iMacs.