The Pew Forum on Religion & Public Life has put up a study on its website that is absolutely fascinating. The data is very interesting. It gives tons of information, not just about what religions are practiced in the United States and where, but also what the practitioners are like. It allows for comparisons of age and income brackets between and within religions.
WARNING: This link may not be appropriate for work. Not because it is vulgar or anything, but because the website is so well put together it threatens to eat away more time than you may want to give.