UR has a very distinct and strong poli sci program; their PhD graduates place very well, and their undergrad grads are looked at as strong candidates in PhD admissions. If you think you might want to do a Poli Sci PhD, I would suggest considering majoring in Poli Sci as opposed to an interdisciplinary major, or, if you do do the interdisciplinary major, play up the poli sci aspect and your experiences within the department (aka UR poli sci department is a brand/affiliation that will be a strong asset to be closely associated with).
For profs that are very well known in what you want to do (i.e. their recommendations would pull a lot of weight at other poli sci depts), if possible, take courses with Stone, Goemans, Powell, Signorino in IR/Comp/Methodology training, and Johnson for Poli Thry. (That said, these aren't the only people you should look to for recommendations - if you hit it off with a prof who is younger or less well known but can speak very highly and personally about you, that is better than a generic from someone in the list.) Also, take lots of methodology in both formal and statistics. Show interest to professors in order to be invited to complete a senior honors thesis.
In terms of going straight to grad school or going into the Peace Corps, I would do the following: 1) If you have a professor that you have a repoire with, set up time to speak with him/her and ask about the profession, how competitive you'll be as a candidate, and elicit his/her suggestions, etc.; 2) take the GREs and see how you do. If you don't do amazing, you'll probably be a more competitive candidate by distinguishing yourself through real world experience, whether it's in the Peace Corps or otherwise; 3) If your heart is set on doing the Peace Corps, do it, and then see if you still want to get a PhD. Once you start/finish your PhD, you won't have the option of doing it. Also, the experience will probably help you a) decide if you really want a PhD and help you clarify what your research interests are (sounds like you're already specific, but some varied experiences will either solidify it or change it).
Hope that helps...