I have hired tons of people and the letter is a must.
In terms o a call - depends on how you present your self.
BIG don't - being very aggressive, demanding to know what is up/are you still in the running, making off hand comments like "well since I have not heard from you I guess I'm not in consideration", and the like.
Best move - if you have not heard anything in a week, make a call to inquire if there is anything else that you can provide the employer regarding your application and simply state how interested you are in the position (possibly reference something you learned that was very interesting about the company) and that you believe you could make a significant, positive contribution at the company and within the bosses team. If the employer wants to talk they will, if not be polite and let them know you are available with any additional information as needed.
good luck
http://www.clevetriclub.com rob reddy