OS X Mountain Lion needs a Mid 2007 or newer iMac, so you can upgrade to Snow Leopard and then to Mountain Lion.
First, make a backup of your files. Then, insert the Snow Leopard disc and upgrade. After that, open > Software Update, and install 10.6.8.
After installing Mac OS X 10.6.8, check that your apps are compatible > http://www.roaringapps.com Also, open > About this Mac, and tell me how much memory the computer has got. You should have at least 4 GB of RAM to get the best performance of OS X Mountain Lion.
Finally, open App Store and purchase OS X Mountain Lion. When the download finishes, the Mountain Lion installer will appear, so follow the steps to upgrade to 10.8.3
Let's make sure I understand about memory
From the "Get Info" window about the Hard drive this is what I have:
Used: 62.55 GB on disk
From the activity monitor under the system memory:
Free: 590 mb
Wired: 246.65 MB
Active: 945 MB
Used: 1.43 GB
I'm not sure how to interpret this data to know I have enough memory to upgrade to Mountain Lion.
Thanks for any reply
It looks like this iMac only has 2Gbs of RAM installed.
No where near enough RAM for OS X 10.8 Mountain Lion.
Your iMac model can take a max. RAM of 6 GBs.
I suggest installing the full amount for best performance of this iMac under Mt. Lion.
Correct and reliable Mac RAM can be purchased from online Mac RAM sources Crucial memory or OWC (macsales.com).
Here is OWC link for the RAM you need.
Just so I'm sure I understand. I can add 4 more GB to the two that are already in there, right?
Possibly, but likely not. Your 2GB that is already installed is probably 2 1GB chips. If I'm right then you would need to toss those and get OWC's 6GB kit which is 1 2GB Chip and 1 4GB Chip.