I think it depends on your school and what courses you take. I am in Neuroscience/Physiology and graduates from my college have said that they do well in medical school simply using anatomy notes from out UG Human Anatomy, etc. I looked at the Neuroanatomy exam for med students when I was taking Neuroanatomy and was perfectly capable of doing it. If you take advanced Genetics, Pharmacology, Embryology, Physiology, Neuroscience, Anatomy, Virology, Pathobiology etc. it will surely make med a lot easier.
What type of Biology are you considering majoring in? Cell Biology? Evolutionary Biology? Microbiology?