For basic text notes you can use Apple's 'TextEdit' app which comes free with any OS install. It's not the greatest app ever known to Mackind, but will read / save as rtf and doc formats, although without a lot of the formatting and styles Word offers.
Appleworks will do a little more, inclusing save as various Orifice formats, but really it isn't up to much. If you get it free (as you do with iBooks I think) then try it for a while, but I wouldn't jump into buying it.
You could also try either OpenOffice or (forgets the name) one of the other cheap / free alternatives.
I guess it depends how much detail and formatting you are going to be having in your notes, and whether maintaining all that between PC and MAc is a high prority.