The one thing I learned is that Apple restricts you in a thousand ways. It came with 10.2.8 loaded. The original OS was 9.2.2. I wanted to load 10.3. There is no one standard MacOS X 10.3. There's OS for eMacs, iMacs, PowerBooks, PowerMacs... In addition, some OS versions are machine-specific!! There are a hundred different kinds of Macs with exotic names - Blue and White, Bronze Keyboard, Mirror Drive Doors.
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The bundled operating systems only work on the mac they shipped with in many cases (in some they don't.) Mostly to prevent copying and use on other machines.
If you purchase a Mac OSX system with it's own box, it has all the neccesary components for all machines.
There are of course the same exeptions in the Mac and Windows world, where some operating systems no longer function due to changes in the hardware making older systems unusable on new machines, and new systems run very slow or not at all on very old machines.
When it comes to Mac OSX compatibility between the newest and slightly older machines, I have understood that this has to do with the change from Openfirmware/Openboot to Extensible Firmware Interface (EFI.) This is the bit that loads the operating system (Most windows systems use the outdated BIOS type ) and therefore new OSX systems can be used on older machines, but not the other way around.
See: [a href=\"http://en.wikipedia.org/wiki/Extensible_Firmware_Interface]http://en.wikipedia.org/wiki/Extensible_Firmware_Interface[/url]
I learned that you can't modify things like you can with PCs. You can't load an earlier version of the OS. If I want to, I can use Win95 on my PC laptop!
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You can load a lot of older systems, the installer will detect the new system if its there and "deny" a downgrade, so you have to erase the old system first (backup!) In some cases there are the aforementioned hardware upgrades making some older systems unusable. There are a lot of ways to modify a Mac OSX system... what do you want to do (besides using an older o-system...)
Good luck with your foray into the Mac world, and if you are uncertain, I have installed windows as a second operating system on a couple of Macbooks and Macbook Pro's recently, and it works well using the boot-camp beta, so there is no need to choose one or the other. (Windows even picked up the iSight camera, bluetooth modules, and all the other bits and pieces...)
regards
-axel