How to tell what version a Snow Leopard DVD is?
You should be able to see the OS version by going to System Preferences > Startup Disk. If it doesn't show in Startup Disk for some reason, you can run the following command in Terminal:

defaults read "/Volumes/Mac OS X Install DVD/System/Library/CoreServices/SystemVersion" ProductVersion


If your DVD is not called "Mac OS X Install DVD" then you'll need to replace that part of the command with the actual disc name.