As far as postgraduate study in the US goes, to my knowledge it is largely PhD and research focused. Although I can only speak for my own subject here, economics, instead of doing a masters and PhD as in the UK, American students progress straight from bachelors to a 5 year PhD programme, where the first two years will be taught and then you move on to the research. All the things about ECs can be thrown out of the window for postgrad, they only care about your academics and connections, and they have to really be outstanding if you are thinking of Harvard, MIT, Princeton, e.t.c. As in it would be extremely difficult for me to get in from Cambridge undergrad without doing a masters degree and coming near the top of the class, doing assistant research stuff and building a good relationship with the academic staff so they can write you stellar letters of recommendation. This could be different for a humanities subject, but I'm guessing PhDs and funding (esp. funding) are even more difficult to get for humanities.
The other option is a masters, which tend to be more applied and lack funding, but this would tie in well with your interests in development. However, you would probably need to get at least a couple of years of fieldwork or relevant work experience. Plus if you work for the right firm, or the right policy department/think tank, they might fund you through such a programme if you return to work for them. Getting experience is something you can start doing at university, but you probably have a good five years before you can think of applying to such programmes. If you are really interested in doing this though, it's good you are thinking about it at this stage.