Why, you may ask, is cooling the case important?  First off remember that the standard PC uses an air cooled heat sink to cool the processor.  The cooler the ambient temperature (inside case temp) is, the better the processor can be cooled.  Now remember the processor isn't the only thing generating heat inside the case.  The power supply, the chips on the motherboard, the ram, the various cards, and the drives all generate heat.  This heat is thoughtfully trapped inside the case.  In the old days the fan on the power supply was enough to keep the internal temperature under control.  Today, the case needs additional ventilation.  Lets take a quick look at what AMD recommends:

 Airflow Guidelines for Cases and Power Supplies (PDF file)

Here you can see that at minimum you need an additional case fan exhausting the air, placed in line with the processor.  The air entering the case should come from as low as possible on the front of the case.  This forces the hot air out of the rear of the case and allows "cool" room air to enter the lower front of the case and pass over the components picking up heat and then getting exhausted from the case.  Most people interested in OC add at least one additional case fan to the front, drawing air in.  This increases the air flow and lowers the case internal temperature.  A real popular case modification (mod) is to add a blow hole at the top of the case.  After all we all know the heat rises and the hot air will tend to collect at the top of the case.  If we drill a hole through the top of the case and mount a fan in it, we can blow the hot air out of the case.  Serious OC's also place additional fans into the case side blowing air onto the processor and cards.

 

This is a Blizzard Case available from the FrozenCPU.com.  Here you can clearly see the added holes and fans.  While many PC enthusiasts do their own case mods there are an increasing number of companies selling already modified cases.  

The growth in OC and in case modifying in general has also spawned companies designing high quality cases with great cooling built right in.

This is the front view a CoolerMaster ATC-200MX case.  For those that would like to know ATC stands for Active Thermal Convection System.  This is the case that I finally decided on using.  It is a very high quality, all aluminum case.  Mounted behind the grill on the front are two 80mm fans that draw air into the case and, cool the hard drives which are mounted directly behind them.  Barely visible in this view is the blow hole on top of the case.

 Here is a much better view of the top of the case showing the 80mm blow hole, and the fan mounted directly behind the slotted opening.

Quick specifications on the case:

3 ext 5.25 bays

2 ext 3.5 bays

6 int 3.5 bays

Here is a view looking in through the rear of the case.  The motherboard tray and hard drive rack have been removed clearly showing the dual intake fans at the front.  The motherboard tray has an additional 80mm exhaust fan properly positioned in line with the processor.  In my case I opted to install a Enermax 451 power supply which has two fans of its own.  One mounted at the bottom of the PSU drawing air into it, along with the traditional rear exhaust fan on the PSU.

So for those of you keeping count that is four 80mm fans in the case,  A 90mm and a 80mm in the power supply.  Total of six fans in the case moving the air (and carrying away the heat.

  

The Enermax EG451P-VE.  The fans in the power supply are thermally controlled and slow down when not needed.  This is a very quite power supply.

NOTE: The grill has to be removed from the bottom fan to fit in the ATC-200 Case.