It is very important that you realize getting a PhD is not going to get you into med school, if your undergraduate GPA and MCAT are not good enough. Graduate school GPA does not compensate for undergraduate GPA. The only program that affect undergraduate grade is Post Bach Programs that are specifically designed for people who need to complete their pre-med requisites and/or improve their grades.
However, that does not mean you don't have to worry about your graduate school grades. It still needs to be good enough for medical school. If they are bad, these grades can hurt you instead. I know it is not fair, but that is how the game rolls. Of course, your MCAT has to be at least average at the schools you are applying to as well. I can't comment much more than that since I don't know anything about the new MCAT.
Admission committees take PhD as a very good extracurricular activity, nothing more than that. So bigger academic schools with research emphasis will notice your application more. At schools that do not have much research, having PhD most likely won't make a difference. That means you still need to have clinical exposure with hospital volunteer and shadowing experiences. In addition, you need to build relationships with people who would write meaningful recommendation letters for you.
In summary, PhD is going to give your application edge for research oriented schools, but it is NOT going to compensate your application weaknesses and get you into med school. Also graduate school is a process that self filters those whose hearts are not in it completely. It is a tough long journey that will eventually burn you out if you don't LOVE research. So if your ultimate goal is to be a physician, I recommend you research other ways to strengthen your application. If you are intersted in others' opinions, there is a short thread on student doctor forum.