Before you buy anything, first make sure your Mac is compatible.
Minimum specs for Snow Leopard. Mac OS X v10.6 Snow Leopard - Read the Technical Specifications
Once you've upgraded you will need to apply the Combo updater for Snow Leopard. Mac OS X 10.6.8 Update Combo v1.1
Remember to repair permissions both before and after each upgrade /update
You can no longer buy Snow Leopard from Apple retail, only as follows. If you are in North America ...
Buying Snow leopard from Apple. Last I read, you must call them (1-800-MY-APPLE). It is $29 plus tax for family pack and $19 plus tax for a single.
Based in Europe or otherwise call your National Apple Helpline … http://support.apple.com/kb/HE57
You should also check the minimum specs for Mountain Lion. Apple - OS X Mountain Lion Read the technical specifications.