What To Wear

This guy I've been seeing is taking me out on a date next weekend. He called me and told me that the place is very formal, and I should wear a dress. He said he will either wear a suit or a tux. I have no idea what type of dress to wear. I don't want something too over the top but I also don't want something too simple. Please help! He won't give me any more clues as to where we're going.