Trust me, you definitley want to take it after your sophomore year. It is doable, I had takend organic chem lab (4 hours a day, 4 days week) for then entire summer, took a kaplan review course, and of course, took the mcat. While I got a decent mcat of 33 (11V10P R 12B), I think that the pain of taking it over the school year was worth it enough for me take it in the summer. You really have plenty of time to work to a decent score. Of course, now that I took bact. and physio, things that I had to teach myself are being taught, and questions that I couldn't have answered are now doable. So maybe I coull've got higher, or maybe not, maybe my grades would've dropped, maybe it wouldn't have. So basically, if you got the those basic courses, just practice up and take it. Trust me, its really nice to not have to worry about taking that MCAT in April.
peace