good luck with the interview- grad school is both awesome and scary. What type of degree are you going for? im working on my dissertation in sociology, hoping to get a job interview this spring, so hopefully I will be in your shoes soon.

as for dress: be professional, but don't go way out of your comfort zone, you don't want to be uncomfortable in your clothes and make yourself more you more nervous or anxious. also, wear comfortable shoes! it's a must.