If possible, "stake out" the office before the interview, sit in the parking lot and watch people coming and going, and see how they're dressed. For an interview, you want to be dressed better than most employees on an average day. You definitely don't want to be dressed more casually than the typical employee.
Dress standards vary widely. My current company is very casual. The hiring manager I first interviewed with told me he preferred, but wouldn't absolutely insist, that his employees wear a shirt and shoes to work. He wanted us to be dressed well enough that we wouldn't get kicked out of a fast food restaurant. I interviewed in a suit and tie, and got the job. But I don't think the suit helped.
I've seen candidates come through here in sharp business suits, or in jeans, and I don't think it has ever made much difference either way in our office. But even in a casual office, it doesn't hurt to be a little bit dressed up for an interview. In a formal office, it may hurt a lot to be dressed too casually.