word is definitely a RAM hog, and firefox can be on occasion... however 2 GB should be plenty for the apps you listed.
as other posters have mentioned, RAM is getting cheaper all the time. if you cant afford it now, in a year or so it'll be dirt cheap to get 4 GB. as long as you dont experiece any slowdowns, you're fine.
if you do ever experience slowdowns, use istat to see how much RAM is listed as "free" or "inactive"
if adding these 2 categories yields a number less than 100MB, it's time for more RAM.