Since heat rises, it's usually best to have the intake fans lower in the case and the outtake fans higher in the case. I like to have one in the bottom front blowing in and one on the top in the back blowing out(usually the PS fan is good enough). This config gives a good airflow. Some people like to go farther and do a case mod and add an intake fan on the side of the case that blows in right across the processor. That works pretty slick as well, in fact Gateway has been doing that with big 120mm fans on some of their machines for the past couple years.
[This message has been edited by Luds (edited 08-02-2001).]