Submit your AMCAS app now, but let schools know that you will be retaking in August. I was in a similar situation last year, as was a friend of mine. We both took the MCAT in April, and then retook it in August to improve our scores. I submitted my AMCAS in June, and let my schools know (via letter or email, I called to find out what they wanted) that I would have new scores to add to my application. Within two weeks of scores being released in October, I was already on my way to two interviews - both schools had reviewed my application and were just waiting for the updated scores. My friend waited until the last minute to submit AMCAS (after the August scores came out) and he ended up being quite behind in the process. I had my first acceptance by Christmas, he finally got his a few weeks ago. Don't wait, especially if your score is decent enough to warrant at least secondaries.