I did a minor in international development studies. One of the things that I took away from there was that the problems of so-called developing countries (and poor people in rich countries as well) are very complex. There have been numerous well intended, philanthropic projects in past, which actually did more harm than good. Really determining what the most effective way to spend your money is, would require years of study.

There are consultants you can hire to help determine this for you. That said, the influence (be it good or bad) you could have with a couple million dollar is so minuscule, that you might as well measure the benefit of it in terms of emotional value, instead of any "objective" measure.