You should ask the students who show you around.
My guess is your chances are good; while you can obviously screw something up at this point, they're interested in meeting you because you're a strong candidate. I know in biology, getting invited for an interview means you're highly likely to be accepted into the program. If you can carry on a reasonable conversation in a quick meeting with a faculty, and not mutter into your soup, you're in.
As for what will be asked, I have no idea. I've heard of professors asking you to solve some complex thingy right there, or drilling you about something in the field, and I had an interview wherein we talked about biking and jogging the whole time.
The best advice I can give is if they ask you something you don't know, don't laugh and go "I dunno". State what you know about the question, and where your knowledge is lacking, and what your best guess would be. I got asked a question about formin structure, and said "All I'm aware of is an FH1 and FH2 domain, that binds to actin, and a ring like domain that straddles cables, but I'm not sure that's relevant to your question" and got a big smile and cheerful explanation.
That's not very helpful I guess; treat it like a slightly less formal job interview, and know how to ask intelligent questions instead of talking about your undergrad experience the whole time.
How many are the enemy, but where are they? Within, without, never ceases the fight.