How to upgrade MacPro OS 10.6.8 to 10.8?

As long as your machine meets the system requirements:

http://www.apple.com/osx/specs/

You should be able to go to the App Store (since you're running 10.6.8), buy Mountain Lion, and download/install it.

FWIW: You probably have more than 2 GB RAM - 4 GB should be considered a more realistic minimum.