+1 to everything Jon said, especially having the extra set of work clothes in your car. They may spring something on you that you may need to change for.
I think dress pants, button-down, and tie is appropriate. If it's hot I would skip the coat, you don't want to be all sweaty. Be well-groomed and act alert and enthusiastic, but not over-the-top to the point where the interviewer decides he wouldn't want to put up with that level of enthusiasm on a daily basis (been there). I've done quite a few interviews as the interviewer (in a professional engineering role), and there were a lot of people that walked through the door and I thought "You're kidding me." A well-fitting, clean business casual is preferable over a suit that doesn't fit that has stains all over it. First impressions are priceless in interviews, and a lot of that impression is made before you even open your mouth.