American Accents

As a Brit, I tend to think of an American Accent in general; I'm vaguely aware of the difference between Northern and Southern American voices, but beyond that it's all one to me.

My question is, how different are accents and dialects from, say, neighbouring states? How much do they vary across one state?

For instance, is there a 'New England accent', or would the difference between New Hampshire and Vermont be clear to anyone born and bred in the US? How about the difference between, say, New York and its neighbours? What about New York City and Albany?

For the most part, natives of this small island speak noticeably differently to those from the next county, and even many cities have their own dialects almost to the point of incomprehensibility, is there as much diversity across the pond?

This isn't purely idle curiosity: my novel is at least partly set in the US, and I'd really appreciate some info about this.