I'll add my 2 cents: I'm a 2nd year med student.
A lot of people will tell you not to take the same classes as an undergrad that you'll get in med school by saying something to the effect of, "They'll teach you all that in med school anyway."
There are a few really big flaws in this line of reasoning. First, while med school is not a great deal harder than upper-division undergrad science classes, the volume of material per unit time is twice what a full-time undergrad courseload is. Second, the level of detail that you will be tested on is greater than undergrad. Just "understanding the concepts" won't work. We actually had a Histo exam question that hung on whether you could remember if it was a 64 or 67 nm turn in the alpha helix that spanned the membrane! Third, experience showed me that the more med school-type classes you'd had prior to med school (i.e. Micro, Histo, Anat, Biochem, Genetics, Cell Biology, Neuro), the easier you would have it, especially your first year.
For these folks (I was NOT one of them!), first year was a review with lots of extra details thrown in. Then, with a better first year background, second year courses like Pharm and Pathophys, which rely heavily on your first year stuff for background, are less daunting, and the bottom line is - you get tested on this on Boards, so the better you have it down, the better your Board scores!
A lot of people will nitpick the details here - schools aren't the same in terms of class difficulty, have fun as an undergrad, maybe the match between undergrad classes and med school classes isn't a good one, blah blah blah - this is a waste of time!
Remember that you'll need to do the equivalent of A-grade work in med school (yes, even with Pass/Fail since you'll need 70% of the top score, which is always 100% just to pass) with twice the volume you're used to, so be as prepared as you possibly can be going in.
How do I know? I WASN'T well enough prepared because I took the minimum science requirements in everything but anatomy and physiology, and in every class other than those, I had to kill myself! In Anat and Physio, I got to do less study, but had to use the time I "saved" to keep up in other classes.
I'm not trying to be negative or discouraging, and I DO really enjoy med school, just trying to give what I hope is a helpful perspective!