Robin,
 
If you want to go into the exhibition design end of museums, I'd suggest a
degree in design not museum studies.  I know several very good exhibit
designers who came out of the "industrial" design field.  Also graphic design
if you are interested in designing the more 2-d things (like labels, brochures,
books) that museums use.  You can always do an internship at a museum or take a
course or two related to museum studies (though it sounds like you already
have with your art history and historic preservation background).
 
I have an MAT in museum education and have been on exhibition development
teams.  I find that usually the person on the exhibit development team who has
some of the same interests and concerns as I do is the designer.  You may want
to take a class in learning theory and museum evaluation because these tools
will be useful when you begin designing exhibitions for museum audiences. Also,
a class in concensus building couldn't help but help! ;)
 
good luck!
 
Lisa Falk
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