haven't read the threads linked by inafuturelife, but i'll just say that yes, yes, yes, this is a good idea, and can definitely improve your chances of acceptance. just briefly introduce yourself and explain what you hope to study, and then maybe think of a couple of simple questions about the program or their work. (not questions that will be a hassle for them to answer, but just something cursory to serve as an "excuse" for emailing them, something along the lines of "are there currently many grad students working in the field of ____?" or "are there opportunities for interdisciplinary work in the ____ department?" ...that kind of thing.)
of course, before you email them it's important to 1) have a clear idea of what you want to study and 2) be familiar with their work.