I live in Providence and I adore Zuzu's and turn to this delightful dress shop whenever I need a dress for an occasion. (They do have some great casual clothes, but anyone will tell you that they're remarkable for their selection of chic dresses). Their prices aren't cheap, per se, but I'd say they're appropriate for the quality of the clothing -- they range from about $80 - $250. There are great sales though!
If you live in the New England area and are looking for a chic outfit, Zuzu's should, without a doubt, be your first stop.