brown has one of the best applied math departments in the country--based mostly on the quality of the faculty (several have won the most prestigious awards in the world in their field). i'm sure you've seen these
Applied Mathematics Department - Brown University Economics at Brown University | Undergraduate Program
the applied math/econ track may be the most efficient combination of your interests.
as an added bonus, both departments are housed in some of the nicest buildings at brown
photoalbum.htm Robinson Hall, Fall 2003