Two intakes in the front, one exhaust on the top, one exhaust on the rear.
Fans on the heatsink should be oriented, so when you look at it, it draws air from the right and blows it out to the left. The entire case airflow should go Right (Intake, front of case) to Left (exhaust, back of case).
I've built in about 6 300s, and this will definitely be your best bet. The side fan is pretty useless unless you're overheating a GPU.