Welcome to the Apple Support Communities
You have to upgrade to Mac OS X Snow Leopard to be able to upgrade to Mountain Lion. Buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard Then, make a backup, insert the DVD and upgrade. Finally, open > Software Update and install the most recent version.
Open App Store and purchase Mountain Lion. See if your programs are supported > http://www.roaringapps.com