Good plan to keep looking. If the job comes, the job comes. No point in sitting by the phone waiting for it to ring.
I think I'd wait a week before calling again. Did you send a thank-you note for the interview? That's always a good move. It shows your continuing interest. I always used to say thank you, tell them how interested you are in the job, and invite them to contact you if they need any further info. The reason I suggest waiting a week is that they have other things to do besides filling this job (and you never know what might have come up), and they may need some time to contact your references.
A week at least seems a good period of time to wait. The letter will show them how interested you are, and beyond that you can seem annoying or impatient.