If you want to do research, do it in your residency and don't do a PhD in medical school. It doesn't help out an application that much, and definitely not worth the time involved. No to mention you'll be 7+ years past your PhD training by the time you complete residency, much less a fellowship, should you choose one.
Now if you already have a PhD - my experience was that when interviewing, I felt that they were encouraging another 1-2 years in residency for research - which I was not interested in. So maybe it might help if you want one of the 6-7 year programs. Then again you'd be 40 before you start practicing.