Summary.
1) A masters in medical science (also known as a special masters program, or SMP) is completely worthless in terms of getting you a job.
2) An strong performance in a masters in medical science is the single best thing you can do for your medical school application. People with good SMP GPAs (including myself) have gotten in with GPAs of 3.2, 3.0, and even 2.8.
3) A weak performance in a masters of medical science program is basically an application killer for medical school. People who had a bad year somewhere like Georgetown, Drexel, or Cincinnati reported that there was basically nothing they could do to rehab their applications.
4) A normal masters degree, though very helpful in terms of getting you a job, is very low yield in terms of improving your medical school application. If you want to improve your med school app, unless you're VERY borderline, you need either more undergrad or a masters of medical science.
5) Finally, just so you're aware, most schools don't care about your performance in a masters of medical science program very much until they have your entire GPA. So you would be in classes throughout this year so that you could boost your app for the NEXT application cycle, not this one.
See the post-bac forum for more info.