i mean, your best bet on this question is to ask one of the professors in your university's history dep't that focuses on economic history... the people on this board are pretty damn knowledgeable, but frankly your profs will know the full scoop with the departments that are best for you to apply to (e.g. let's say you want to work with a particular prof at harvard; your prof at your current institution will know if that person is imminently retiring and may well leave in the middle of your ph.d program).
it seems from your post that you're probably a junior in college right now... if youre coming straight out of undergrad, the profs at places you're applying to want to see that you have taken a pretty fair number of classes in the area you want to specialize in (versus people who apply some time after college and have had time to do more research and possibly publish a bit, so you should probably establish pretty soon your particular interest. rockeater's advice is really good -- one of the things admissions committees are impressed by is how specific a statement of intent is in terms of what the applicant intends to research and how much he/she already know about it.
also be forewarned that if you do not already have a solid reading and writing ability in applicable foreign languages (if you choose a foreign area of concentration), you're going to have a hard time getting into a ph.d program, where they expect you to be able to hit the ground running in foreign language research if you need it. harvard, for example, expects its applicants to have reading ability in two languages other than english (which is no mean feat esp. if you want to do anything asia-related).
plus, the list of grad schools you have right now is nuts -- 20 is too many -- but again, once you actually have a sense of what you want to research, you'll know where the best places to apply are.
sorry if this sounds harsh. which university are you at right now?