Slant Fin just released an updated version of their heat loss calc. It's a very good place to start. Here's a link to the article.
http://www.prweb.com/releases/slantfin/heatlosscalculator/prweb9915043.htm
http://www.prweb.com/releases/slantfin/heatlosscalculator/prweb9915043.htm