Hi,
Have a look at the Uno on the hardware page (http://arduino.cc/en/Main/Hardware). It's the standard from which the others are derived, has a good range of shields to help the beginner get started quickly, and from what you've said should well cover your immediate requirments for IO pins and memory. Once you get started you'll have more of an idea what your next one will be too 
Cheers ! Geoff