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
Note. You can no longer buy Snow Leopard from an Apple retail store, only as follows.
If you are in North America ... Last I read, you must call them (1-800-MY-APPLE). It is $29 plus tax for family pack and $19 plus tax for the individual use version.
In Europe or otherwise … call your National Apple Helpline … http://support.apple.com/kb/HE57