The biochem I took in medical school is very different from undergrad, although there are overlaps. Undergrad included a lot of memorization of structures and details focusing on mechanisms along entire pathways, whereas medical biochemistry tended to focus more on regulated/rate-determing steps along with pathologies associated with such pathways.
Being that I took undergrad biochem almost a decade before medical school, I can honestly say that it helped me minimally in medical biochemistry. My advice is to only take undergrad biochem if it fits one of the following criteria: it is a mandatory course for your major; schools you are applying to require it ("recommend" isn't enough IMO); you are in the process of making a GPA comeback and must demonstrate competence in higher level sciences; you just have an interest and can't stand the thought of passing it up. If you do not fit that criteria, you may be kicking yourself in the rear when you're memorizing the light and dark phases of photosynthesis wondering "why, WHy, WHY?!?!"
Spend the credits taking anatomy, neuro (if interested), or any other subject of interest. Or, better yet... like NeuroSpeed mentioned, enjoy your free time. Because come August... it's GONE!!!
EDIT: just wanted to add... biochem is not THAT hard in med school. plus, it's not considered high yield on Step 1 (which holds A LOT more weight that your preclinical grades).