I can't upgrade from Snow Leopard to Mountain Lion
I have a Mac Pro (1.1.1) running Snow Leopard (Mac OS X10.6.8.) I want to buy and upgrade to Mountain Lion. I've followed the instructions on Apples webpage http://www.apple.com/osx/how-to-upgrade/ and have (I thought) met all the criteria to upgrade. However, when I go to the app store and attempt to purchase Mountain Lion, I get the message:
We could not complete your purchase. OS X Mountain Lion is not compatible with this computer.
Where do I go from here?
Mac Pro 1.1, Mac OS X (10.6.4)