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
I think it probably is as a mid 2007.
Neil gave you the number for North America. In Europe or otherwise … call your National Apple Helpline. Find it at … http://support.apple.com/kb/HE57
Once you have bought and upgraded you will need to apply the Combo updater for Snow Leopard. Mac OS X 10.6.8 Update Combo v1.1