Most likely your MB was in the area right between the transition to Leopard and the phasing out of Tiger. As far as I know there are no OS X upgrade discs. So chances are you have two OS discs. Which is pretty sweet in my book. I'm sure you can upgrade to leopard with the disc, but I have heard nothing but problems with that. I would suggest a format and re-install with leopard but I would wait for a second opinion.

Also is this similar to what you are talking about?

Is my bundled Leopard disk just an upgrade?