Late reply but--you might want to look at MA programs in the UK (i.e., Courtauld, UCL, St. Andrews) that have good reputations and are more lenient about coming from a non-art history background. However, if you can make a good argument for your switch into art history and emphasize how the skills developed in your undergrad will contribute to your success in grad school, you might be able to get into the US MAs that require a stronger background in art history.