There are some programs you can download that count how much you're downloading, but I can't remember what they're called now. Sure someone will post a link though.
However, if you just browse the internet, check your emails, and do general things like that, then you probably won't use over 2GB a month. (unless you're emailing large files). The only way you'll use over 20 or 40GB or so is if you download music or films from P2P clients, or places like iTunes Music Store.
Try and get a non-capped one if you can anyway, they're better. But if not, and you don't use the internet much, then it doesn't really matter.