I personally did not wear a suit, but just khakis and a dress shirt. Many of my friends in the program did not even have a face to face interview, but rather it was over the phone! The NIH is a very casual place, as one of the posters said, but if you really want a suit, it's not going to hurt.
As for non-attire things, I would definitely read up on the PI's research page on the NIH website. Personally, I was asked some basic questions by the PI about his research. Know why you want to go into research in general, and the PI's lab specifically. It also wouldn't hurt to brush up on some lab techniues like PCR, cloning, genetics, etc. because I've heard some PI's like to "quiz" you a bit on the spot.
Maybe I'm scaring you too much, because my impression of the IRTA program is that if you are invited to an interview, you will most likely be accepted. A PI doesn't invite too many candidates to a personal interview like that for a post-bac position.