You should take the MCAT when you feel prepared to do so. The only downside to taking the MCAT too early, is that the scores are only good for a certain period of time (varies by school but typically about 2 or 3 years). Good luck!
a lot of kids i met who are very bright weren't quite ready to max out their scores in august. a year of school actually really helped them (and me) to think a bit more critically etc... which i think is really more important than book knowledge. and oddly enough, i liked studying for it much better in an academic environ with everyone else than doing it all by myself at home (completely personal pref). this isn't to say you won't do well, you just might do much better in april; just a thought. goodluck regardless.
As long as you have taken all of the courses necessary, I dont see any problem with taking it in August. If you havent finished physics or organic, definately DO NOT take it! I think taking it in august will give you several advantages. First of all, if you do well, you wont be stressed out about it all throughout your junior year. From experience, that is a BIG plus. Also, if you need to retake it, you can do it next April, and you won't be behind in submitting your application. However, if you don't feel completely ready or prepared to take it in August, I would recommend waiting till april.
All I have left in premed courses is Organic II and lab. The lab won't matter as far as the MCAT is concerned.
My number 1 school is out of state, so I have to have a good MCAT score...I guess I need to take it in August, then. And then perhaps retake in April, if necessary. I had like a 27 composite score without studying on a practice test. So perhaps with a couple months worth of studying, I could improve that significantly.