Any upper level biology or chemistry courses will demonstrate that you can handle hard courses. Many schools suggest that you have biochem, genetics, calculus, etc in addition to the standard one year of gen chem, o chem, bio, and physics.
I disagree with a previous post claiming that other courses (like biochem) are not helpful for MCAT preparation. When you advance through medical training you'll realize that getting the correct answer or making the right diagnosis is more about word association and pattern recognition than actually knowing the facts (though obviously important). This is why clinical experience is the bulk of medical training for doctors as well as PA's, NP's, & RN's. With that being said, taking a semester of biochem gives you 15 weeks of material exposure in addition to the usual MCAT prep. When you start medical school, all of your important learning will be based on repetition, rather than endless reading. Another benefit to taking advanced courses is that you learn the material much more in-depth, which could possibly allow you to answers questions more quickly (and more accurately) on the test.
-senior medical student / admissions committee interviewer