The physics of it:
Heat can flow, we all know this. But if the air in the case is stangnant its heat is going to equal out with the heat of the hardware. When this happens no heat will flow. Because no heat is flowing off of the hardware the hardware will heat up some heat will go to the air, so on and so fourth. Bottom line is the temperatre (which is not the same as heat) will increase. Cooler air will be able to absorb more heat (heat can be thought of as kinetic energy on an atomic level, but I like to think of it as a loss of kinetic or electrical energy to the environment) then the warmer air. So ideally you want to exaust the warm air asap and replace it with cool air to keep heat flowing from the hardware. This is the basic principle with any air conditioner, take the heat from the inside of the system and move it outside of the system which will therefore decrease the temperature inside of the system. But if you have all of the fans blowing in you are just pushing more air on top of warm air and that warm air will be compressed up against the hard ware. Sure air is going to leave through the few holes within the case, and sure the cool air is going to take some heat from the warm air, but I find this method less effective. Ideally the case will have the warm air flowing out at one end and the cool air flowing in at another, make the case a wind tunnel if you will. I just feel that having more output fans is more effective then having more input fans.
Heatsink fans sort of go both ways, I have seen heat sink fans that blow air onto the sink and fans that flow (or rather pull) air off of the sink so that new air can move in.