I suppose it is up to you but by far the majority of interviewees that I see around campus are wearing suits (at least 75% IMHO). I would say that looking professional at your interview is important. I cant imagine that you would loose points for not wearing a suit, but wearing one certainly will not work against you.
One one of my interviews one of the guys wore a sweater....I didn't think that was very professional. All of the men from admissions wear suits, so I think it's a good idea to wear one as well.
I would definitely advise you to wear a suit. Don't give your competitors an edge if it can be avoided. Look as professional as possible. A navy suit with a red tie is always safe.