I got my MS seven years after undergrad when I decided to apply to med school. I'd say having a masters helped in two distinct ways.
The first way, we'll call it knowledge base, obviously depends on what courses you take. As a graduate student I took a neuro course, biochem, applied physiology, etc which really helped. If med school is your goal, take courses you see when you get there.
The second way it helped is a little more abstract, but it has to do with critical scientific thinking. Choosing and preforming a Thesis requires you to read a lot of relevant papers and dissect what they're saying. A good advisor will also help you make the transition from reading for learning and "reading for thinking".
Oh and like someone above said. If you can apply to med school now then forget the MS. But if you need an application boost, well...it worked great for me.
good luck