You dont /need/ a laptop. Only about 10% of my class bring theres in. I use mine to write note, using a tablet, on the powerpoint slides my teachers use. They make them aviable at the start of the lecture so I download them and scribble away. I think its really useful.
If you do get a lappy, you dont need anything powerful at all, but the keyboard will be very important. Dont get a netbook.
Everyone learns differntly, so its done to you. Just remember 15.4" laptops are heavy, they dont feel it but after a whole day, it gets to you.