Hi, Isaac -
That model iMac is suitable for OSX versions 10.4 and earlier. It can not use OSX 10.5 (Leopard).
If the OSX install disks you have are ones that came with a different model Mac, even an iMac, they are not usable; such are model-specific to the model they shipped with. You will need to use a retail (separately purchased) disk set.
In particular, the retail disk set for OSX 10.4 comes on DVDs. If your optical drive is not DVD bootable, you would need to get those on CD. You can get such, as well as older retail sets, from sites like this one -
http://store.yahoo.com/hardcoremac/index.html
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You have sufficient space available on your hard drive. OSX will need 1.5 to 3.0GB of disk space to install, depending on version and customization of the install.
In addition, OSX runs best if it has 5GB of free space on the drive, though it can manage with less (figure on a minimum of 3GB free space). It needs this space for its version of Virtual Memory and for such things as its automatic on-the-fly defragmenting of files.
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If you do not know for sure whether the firmware on the machine has been updated, go ahead and update it - if the update is not needed, the installer for it will so state and refuse to run further. You can download the needed firmware update for that model from this Apple KBase article -
Article #HT1395 - Firmware Updates
Do this before attempting to install OSX.
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Your iMac has insufficent RAM for any adequate version of OSX, such as OSX 10.2 or later.
OSX 10.2 and 10.3 require a minimum of 128MB of physical RAM; a more realistic minimum is 256MB.
OSX 10.4 requires a minimum of 256MB of physical RAM; a more realistic minimum is 512MB.
OSX runs better the more RAM it has available. Your machine has two RAM slots, and can use modules as large as 512MB; this gives it a max capacity of 1.0GB of RAM.
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OSX is not an upgrade to OS9; it is a different family of OS altogether.
OSX and OS 9 are designed to co-exist on the same drive; and, on machines such as yours using OSX 10.4 or earlier, OS 9 can be used within OSX as Classic. In this mode it provides an environment in which many OS 9 programs can be used without needing to restart the machine using OS 9 as the boot OS.
The importance of this is two-old -
• The default install of OSX will not remove your install OS 9. Instead, it will be installed alongside OS 9.
• If you remove the install of OS 9, you will not be able to use any of your old OS 9-only programs. Since OSX has a different base than OS 9, OS 9-only programs can not run in it (they don't speak the same language).