I agree with Dawnie, especially about sending a card right after the interview. Since it's already been a few days, I also agree that you should go ahead and call - with confidence. It never hurts to thank them again for their time, and inquire about the decision. (Keep in mind that this time of year is sometimes vacation time; they might have been out of town....)
But - be prepared, just in case you don't get the answer you expect. If this one wasn't meant to be, there will be something even better out there for you. Keep trying!
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Opinions are mine and never to be taken as legal advice!