Talking a bit about your proposed research sounds promising - that might make your application stand out from the others, and show you're going to make serious academic use of the experience. I think that if you are going to be writing about emigration to America, it makes very good sense for you to have spent time there. You could say there are specific courses (maybe one that's offered in all the universities) that you want to study in preparation for your dissertation.
Have you a particular interest in America? (You plan to write about it, you want to study there, etc.) If so, tell them.
Even if you're not involved in stuff at university, I think I'd still mention my interests outside college, to show what a well-rounded and capable person you are and give a flavour of your personality.
What do you think is your strongest argument (though you should try to cover them all): academic performance? motivation? personality?