From TheBestCaseScenario

..is a concept regarding the property of an fluid (gas or liquid) to flow. What we want in computers is to have unimpeded airflow, which means that air shouldn't have much difficulty reaching a point in the case where it should be used for cooling purposes. A good airflow scenario is when you have air moving from the front of the case, easily and constantly, going inside, without hitting wires and cables and lose its speed and apparent "pressure", then and only then hitting the heatsinks in order to lower their temperature, then leaving the case as fast as possible through the back or top or side. So, good cable management and a good assortment of big fans driving air inside and outside of the case is essential.

When modding, you can take the responsibility off of the manufacturer's shoulders and in your own hands: prepare, think it through and design a case with a good airflow inside.

[edit] Variables

Variables to consider are the speed of air, the impedance of the box, the volume of the box and the possible way of air moving inside.

[edit] Air speed

Air speed relates to the intake side and exhaust side, but, related to the fan configuration, it mainly tells us how much air you can move and how fast. The faster, the noisier things can be. More speed in one place can be overkill, as thermal transfer is non-linear function and if you use a very powerful and big fan there is no way you can get temps lower than a given, calculable number. A more evenly distributed airflow could be better, but this kind of motion isn't very turbulent, so there will be layers forming around components, hindering the good transfer of heat. So, the speed and shape at which air moves is important and the results vary and are dependent on the configuration of fans/box you have.

[edit] Impedance

A long path for the air, even if it is straight can be harmful in terms of noise and vibrations, but good in therms of heat-management. The concept is simple: if the air can go in a straight line, it will do it more effectively. If it has to corner a lot, it will lose energy and momentum when hitting the corners. And if the corners aren't in need of cooling, we lose precious air power. A smaller box with a nice directed air path can be more efficient than a 2 foot monster with lots of components inside that block the air.. Take for example the mounting of 4 hard drives in a case that offers little space between them: the air that can get through them will get warm and heat things inside the computer instead of cooling them. Also, having an intake at the bottom, front of the case, and the exhaust up and far away, divided by long video card is thermal-suicide! The air path is severely impeded and cool air has problems in flowing and cooling.. Therefore, a less cluttered case is better and one with intakes and exhaust positioned in such a way that air can be directed fast in and out, without taking detours, is even better.

[edit] The volume

Bigger cases need more fans to move and change the air trapped inside.. It isn't necessary, but it is somewhat related to the refresh rate of old CRTs: the bigger the screen the harder it is to refresh it with a higher speed. Controlled air circulation is essential, because even if a big case offers the room air needs to take wide detours without losing precious cooling power, there night not be enough power to keep the air circulating, so, again, air will enter and leave in an inefficient way. So, the volume of the case should be considered: fans need to be positioned in such a way to cover the entire volume, not leaving dead spots where air can stay and take a brake. Imagine this in a liquid cooling loop: 70% of the water passing through the main water block decides to stay in a pocket and not help.. and getting warm too.. although this is close to absurd, imagine a water block with a big "reservoir" under the in and out port, where water can stay and warm up.. never being recycled by the incoming water, because it has no push in that zone.

[edit] Moving the air

Air can be moved using fans, axial or radial, or other methods.. As long as there is enough of it to be properly used by whatever heatsink is used, it is ok. When there isn't enough fresh air problems appear and temperatures as high as 80-90 degrees could appear. Of course, your computer might slow down or die, but you would never blame the case for killing the cpu, right? If you want to have proper airflow, a wall of fans or air moving doo-dads on one side isn't going to be enough, of course.. You need 2 :D Air should be moved preferably silently, fast and in a uniformly turbulent fashion. Sounds hard, but all you need is the biggest fan you can fit! This applies to all places, but remember to keep a certain balance.