I disagree wholeheartedly with most of the nay-sayers on here who claim that a non-SMP masters degree will not help you get into medical school. I attribute most of my success in the application process to my M.S., and the opportunities that came from being involved in my program. I did not do an SMP. I did, however, take mostly science courses. There was a bit of both MPH- and policy-related coursework, but the vast majority was composed of statistics, biochem, immuno, micro, etc. (It is true that your graduate GPA is counted separately from your UG GPA. Still, if you can maintain a 4.0 in grad school, this will certainly reflect well on your application.)
Another important consideration is the atmosphere in which you will be working. I had a great experience in graduate school, a very low-stress, stimulating, and healthy class environment (which is much more than I can say for most SMPs), and incredible classmates and mentors. I was also able to stand out without being labeled a "gunner." Only a tiny minority of people in my program were there with the intent of ultimately going to medical school - most were PhD track, or graduating with M.S. to work in government or industry (I actually started out as a 1st-year PhD track student in Immunology & Micro). Many of my classmates were exceptional people with diverse backgrounds (career first responders/fire dept, DHS/DHHS/FDA, TIGR, etc.). Only a few were fresh-out-of-undergrad - most were older, with at least a few years of work under their belt. In terms of class composition, the SMP at the same school was a polar opposite.
SMPs are incredibly high-stress environments - perhaps even more so than medical school. I do not thrive in such cutthroat surroundings. I had to work hard and stand out in my program, but I never, EVER felt like I was competing against anybody but myself. I had plenty of time to volunteer at the hospital, shadow doctors, go to journal clubs and department talks, and actually have a life. And the connections that I was able to make led to some pretty compelling reference letters.
I don't understand why some people are so quick to dismiss M.S. programs as "neat EC"s. Had I opted to do the SMP at the same school, I probably would have ended up exactly where I am now.