Start with the Duemilanove, once you grow out of that you can always move up from there, plus then you will understand the different types of Duino's better and probably have a firmer idea of what you want to accomplish. 
Start with the Duemilanove, once you grow out of that you can always move up from there, plus then you will understand the different types of Duino's better and probably have a firmer idea of what you want to accomplish.