[Aqua_Geek]

If you have Lion, go to the Apple menu -> About this Mac. Then click on the "More Info…" button. The model year should be listed under the model, on the right.

If you don't have Lion, I think you can still get that info from the window that comes up when you click "More Info…", but I forget where (and don't have a machine running Snow Leopard handy ATM).

EDIT: The required models are listed by Apple (linked in the article): http://www.apple.com/osx/how-to-upgrade/