My two cents on the extra undergrad classes...
In my opinion the subject matter we have had first semester is not really any more difficult to understand than what you encounter in upper-level sciences in undergrad. The real difference is the volume of information you are expected to retain. We were told that a med school semester is roughly equivalent to taking 25hrs of coursework in undergrad. So, the problem really becomes how the heck do you find enough time to make this all stick in your brain. That's where having seen some of this stuff before really helps. If you have already seen the superior mesenteric artery in undergrad you spend less time now searching for it in your atlas. (or your smelly cadaver's mangled abdominal cavity) Likewise, if you already have a good understanding of transcription, translation, and protein synthesis you are going to spend less time trying to get a grasp on that in biochem.
So, in my opinion if you have the opportunity to take human anatomy (especially with a cadaver), cell bio, embryo, physiology, genetics, or histology then do it. Now, if its going to be some kind of harship to take these classes then don't stress. You will still be fine come med school time. You just won't have the benefit of having already seen this stuff once, or twice before.
One class I would highly recommend you take is medical terminology. Wacky medical terms are already everywhere in the first semester, and I'm sure its only going to get worse when the clinical years come along. Understanding the meaning of the names of things in anatomy is very helpful when you are trying to remember what and where they are. And although we are in the "basic science" years we still talk a lot about diseases, procedures, symptoms, etc... So, when you hear, or read, things like cholecystectomy, hydronephrosis, perivesicle, or hepatomegaly its nice to know what those words mean, or at least be able to figure it out.