How do I update from 10.6.8? "

I have an Intel Mac Pro (2 x 3 GHz Gual-core) with 4 Gb RAM. It's running 10.6.8, and I'd like to upgrade to 10.8, which is purchased in my Apple Store account. But when I hit "download", it says "Mountain Lion is not compatible with this computer"! That can't be right, can it? What's the right way to upgrade - do I need to burn a DVD of it or something like that?

Also: I have a clone disk, and would like to upgrade that one first - is it possible to install it on another disk rather than the one I'm booted off of now?