Great article on fan placement and the effects on cooling: http://www.bit-tech.net/hardware/2012/02/10/the-big-cooling-investigation/1
For a case using 120mm fans:
1. If using (2) fans, then rear and side placement is the best.
2. Using (3) fans, then top, side and rear is the best.
3. (4) fans, then (2) front, rear and top will create good front to rear flow. Placing a single fan up front does not provide sufficient airflow to create good front/rear circulation.