It's far from necessary, but it's kind of nice to have a car. If you're looking for cheap rent, going off College Hill, over by Federal Hill, can be pretty affordable, if a little sketchier - okay with a car. Agreed with Ezzy, though the East Side and Fox Point are wonderful places to live, full of other college students, and the Grad Center Bar is fantastic. When I lived on the East Side, I paid between 450-550/mo.
So, geology shares its building with chemistry, but I didn't really interact with chemistry all too much. They do some pretty interesting stuff, from what I could tell, and actually have some nice facilities - the geochem building is 2 floors of geo and 3 or 4 floors of chem, tons of labs for teaching and research. I didn't have many chemistry major friends, though, so I can't provide any juicy insider perspectives. I don't think Brown is particularly well-known for chemistry... like many departments at Brown, chemistry is decent, but not famous. My perception of the chemistry classes is that they can be good, when they're not too full of premeds, in which case they're a chore. But that's just undergrad.
Easy access to a Boston is a huge plus. And yes, the food is really, really good in a variety of styles, especially once you start finding the little hidden gems.