errr, yeah.. Don't worry about the mcat until you've taken all the prereqs. Your best way to begin preparing for the mcat is to put a ton of effort into learning the material the first time in class, so review is actually review and brushing up on topics.
That said, it is never too early to start prepping with books because some prep companies, like examkrackers, only present the material that will (most likely) be on the exam. It's just that w/o the prereqs, you likely wont get as much out of the review stuff. I have friends who review with the prep books as they take the courses... They say it helps them hone in on the stuff that will be important when taking the mcat. Sounds like a good idea..? But im not sure.
In terms of best mcat prep: I've heard examkrackers is pretty good. One admissions lady was talking about the berkeley review as being the best because they only work on the mcat.

I personaly really liked the princeton review when studying for sats so i might pick it again, in addition to ek.