I think if you're in school for four years, you should have more than enough time to get a decent background in biology AND be well-rounded with language, writing, public speaking, or whatever other laid back/interesting classes you want to take.
Anecdote: At a medical school interview, I sat in on one of the first few classes next to a girl that was completely clueless about what was going on. Turns out she was a history major with very little science background, and she found just the first week to be a strain. The material being REVIEWED in class that day was a core principle of biochem/genetics: rna --> dna --> protein. For many bio majors, this is information that is almost taken for granted because we've heard it so many times before. Honestly, it made me wonder what she was doing there when that's something she probably should have known even for the MCAT. If it means anything to you, this was at Columbia.
If you're sure about medical school, put yourself ahead of the curve. Biology is not hard, but as someone already mentioned, it sure does help when you already heard it, even if it was just once, before. Reviewing material is ten times easier than learning it for the first time. Even if you're not a science major, try taking one science class a semester. You don't have to plunge head first into anatomy, but every bio classe will cover a few basics that will help your general understanding of the subject. You can use your other 12-15 credits for whatever floats your boat.
Blah, what a long post. I have no life.