This could be a bird in hand situation--just remember, getting into PhD programs is such a crap shoot that there are no guarantees that an MA from Harvard will improve your future school--unless, of course, getting the MA will drastically improve your application in a year or two. Keep in mind too whether the MA is a one or two year program. If it's a one year, and you want to enter a doctoral program the fall after you complete the degree, you'll have to apply after only a half-semester of course work. Even if it's two years, that gives you only one year to impress and get new references.
I'd also like to voice my solidarity with those here who discount the notion that a non-top-ten program will hinder your future. [Though they may have some merit--see good articles about this at the AHA website-- USNWR rankings tend to be widely distrusted by those who actually know things about the internal workings of departments. And, if you are going to go by them at all, the sub-field rankings are probably much more helpful because that's what people care about. If your institution is highly ranked overall, but terrible in your field, rankings don't really matter. At least that's my take. Others?] Firstly, while nothing is guaranteed, there's a litany of people at elite places with degrees from non-top-ten, yet very solid programs. Yale and Brown, for instance, each have young faculty members (earned PhD's in the last decade) from places like UC Davis, UNC, UVA, and the like. More importantly, though, is what your placement goals are for the future. People forget that there is an incredible range of universities in this country between Ivies and schools no one has heard of (which may well be quite lovely places to work at!). Do you anticipate wanting to research or teach more? Or both? Do you want to be at a large state school, a regional private school, a liberal arts school? Perhaps you don't want to be in a high pressure or super competitive place, or want to work relatively normal hours. For those of us in the academic bubble, it's incredibly easy to think that good educational experiences exist only for the tiny percentage of people at the nation's elite universities. And if you admit to yourself that the chances of being an earth shattering academic and piling up awards is quite slim, then you can relax and go about the business of being a decent human being who happens to work as a historian for a living.
It so happens that I recently completed a masters degree at an Ivy, but received rather different advice from my professors--they all encouraged everyone in my cohort to apply to a range of places based on fit, beyond the ivies. From speaking with professors and friends at several universities, my assessment (and it may well be incorrect) is this: An ivy league degree will get your job application looked at. And yes, some schools out there will prefer the ivy/top-ten candidate because the name seemingly enhances the stature of their program to outsiders. But at the end of the day, it comes down to your work. While equal candidates from top-ten and non-top-ten places may not get the same first jobs, I'd say that, much like in undergraduate, though more people from top-ten places may get jobs in a given cycle, top students at non-top-ten schools will get jobs as well. So basically, at the end of the day, go to a school with a good fit, where you'll be well trained, and stand out.