Amy, I don't recall how I dressed [probably somewhat well], but you should mainly expect that he or she will talk to you about what you wrote on the essays. I had only 2 other interviews, and this one was somewhat less intimidating to me because he really genuinely talked about things you probably already have good ideas about. If you noticed, the UC essays are actually just informative, tell about your goals in college, etc. So your dialogue with this professor can't be too strange in nature!
Maybe ask the professor what he teaches - usually you had a slight say in who interviews you. Try to ask the professor's perspective on the school. I mean, professors at Berkeley are REALLY RIDICULOUSLY overaccomplished people whom even the number 1 or 2 undergrads from top schools in any given major can't necessarily hope to match. You should treat this individual with great respect and enter with the attitude that it's an honor to talk to him/her, and that this could be great info about the school, and you'll do wonderfully.
Any other questions, I'll be happy to address. =]