well, judge how much space you will need by how much you have now and how much you have used. obviously the 120 is 40gb bigger, if you think you would use the extra 40Gb then go for the 120. I personally would buy the 120 because they arent much more expensive than the 80's.
as for partitioning - i wouldn't, because you might want to change them later and unless you have partition magic you won't be able to change them without reformatting your hard drive. instead of partitioning - just make another folder, it does exactly the same thing, and then you can change it around as much as you like.