The best Push-Pull effect is to have a wind tunnel effect. A higher pressure CFM fan should be located at the Pull (Rear) side. In short, the difference in air pressure movement will create Vacumn Pulling like effect to expel the hot air to the back case fan. In general, that's to say :

Pressure: (case rear fan) > (pull fan) > (push fan).

Then it will create a gradual pulling air movement effect.

If you are having a rear 12cm case fan,and you intend to use a single fan configuration, it should be of a higher pressure than the 1 placed on the HSF. To increase air movement efficiency, a semi closed shroud should be constructed from the rear of the HSF directly to the rear case fan.

This post has been edited by kucingfight: Mar 5 2007, 12:11 PM

U've got 2 purposes:

Seeking the highest possible OC (provided the other parts are not limiting) on Air Cooling,

and

seeking quietness @ stock/mild OC with Passive/Low Running Fan.

To add, it has got more to do with pressure difference, which of course it's related to the speed of the fan. As said by Shawty, the configuration has to be a vacuum suction. Meaning, the rear fan pressure has to be greater than the one on the HSF.