Hi~
Definitely wear the suit. And look sharp; don't drink the night before, 8hrs. sleep, shave, etc., etc.
I had two interviews a few weeks ago (got accepted into both); one was with a panel of professors and a writing sample, and the other was one-on-one with a professor. Either way, be polite and respectful, but don't be too formal to the point where it's q+a; make it conversational.
Both sets asked me why I chose that school, why I wanted to be a PT, where I saw myself in 10 years, what makes me tick, and what sets me apart from other applicants. For example, running makes me tick because I can feel my anatomy at work (I'm a big anatomy buff). If you have a particular life experience that influenced you to do PT instead of ie. med school, explain that. So soul search a little, write your answers down, but don't practice them to the point where you sounds like you're reciting something.
Also, be excited. Nothing turns people more on than an enthusiastic person.