Whether you need to take science courses beyond the core requirements really all depends on you -- how your mind works, the best ways for you to learn new material, how much time you have. Certainly it's not true that you MUST take biochem,physiology or genetics in preparation for the MCATs. I am living proof that you can take the minimum required courses and still get an excellent score (I'm also living proof that's it's possible to have a 4.0 science GPA and an over-40 MCAT and get rejected without interview by Harvard and Hopkins -- but that's another story!)
The best advice I can give you is to get hold of practice MCAT tests and see whether you think your courses have given you the information you need. Example: most of the practice tests I looked at had physiology questions that I couldn't answer, but I didn't have time to take a physiology course. Instead, I studied the physiology chapters of my General Biology textbook (chapters we'd never covered in class), and that worked fine.
The same goes for prep courses: there are those who will tell you that they're critical to a good score. They're not -- it's possible (and cheaper!) to do it on your own, although it helps if you can get your hands on some of the prep-course material -- I had a friend who passed his TPR stuff on to me, and it was a great help. Still, if you want the materials, the structure, and the discipline that a prep course can give you, then by all means take one.
So check out some practice tests, think about what YOU need, and take it from there.