Depends on the specialty you'll be applying to and the competitiveness of individual programs within that specialty.
All specialties have certain programs which are more competitive than others. For these, research would be an added bonus. More competitive specialties, especially those at high powered research institutions, emphasize research more. However, the type of research which gets you more than a glance is that which results in a publication. Simply spending a few weeks in a lab, or even a summer, which only results in a mention under Extracurriculars isn't likely to make a significant difference.
IMHO spending an extra year doing research is not likely to make a significant difference for most specialties and most programs. Instead of committing yourself ahead of time (ie, assuming you will get accepted to and attend Duke or other programs like it), consider going to whatever medical school you most fancy and if you still find that you might need some major research to bolster your app (ie, for fields like ENT, Ophtho, Derm, etc.) take a year off - most medical schools will support you in doing so, you needn't be in a formal track to do so.
Best of luck.
