I recently bought an Intuos 2, it's 8" X 12". I love the idea of being able to draw truly freehand using more fluid movements than a mouse but there is a terrible hand/eye coordination problem using a tablet, I think you should try one out for a while before you buy one. I find mine most useful for small bits of photo retouching and editing rather than out and out drawing.
It was quite expensive, about $500, they haven't been very forthcoming with drivers for OS X (still in the Beta stages).
It might be worth considering the Cintiq range they do as well, (although these are mega expensive) the ability to draw directly to the screen would be so much more useful for me and I kind of wish I'd saved my pennies and gone for one of those.