I think the OP is talking about a graphics tablet like
http://www.wacom.com/index2.php
to use with a notebook that the OP already has.
As someone who has both a tablet like that and a tabletPC, I'd say that it is difficult to take notes with a graphics tablet. I tried to lecture with it... and it's not very intuitive, in my opinion, because you can't see the ink under your stylus... You have to look at your screen for that. Admittedly, it is a low-cost alternative to getting a TabletPC.
In place of your laptop, a tabletPC (in a "convertible" rather than "slate" form-factor) might be better for you.
Since I haven't taken a class in a while, I don't use my tabletPC for notetaking... although I do use it for my own private notes and for long derivations, as well as lecturing in all of my classes. In seminars, I might use it if I want to refer to a related article on the topic.
While pen and paper is often more efficient, there are advantages to having ink that can be edited (moved, recolored, rescaled) and searched (as handwriting-recognized text). In addition, there's less paper to carry around.
OneNote has a feature that might be useful... you can record audio, which (I believe) can be somehow synchronized (with automatically generated timestamps?) with the ink you write. So, there's more context for your handwritten notes.
Some tabletPCs are not as expensive [compared to similarly-featured laptops] as they used to be. For example, the HP tx2z is an example [with deals going on now for under $900:
http://www.shopping.hp.com/series/category/notebooks/tx2z_series/3/computer_store.