Although I'm not going for a PhD in american studies, I am in English lit. In my research, I've found that most humanities programs look at your verbal scores more than your math. A 720/6.0 is great in verbal, and your math is at least better than average! I know that you should get at least a 700 in verbal to get into top programs (and don't forget about your gpa, letters of rec, etc. - everything plays a role). I think that you'll do fine.