Replace the case cover. The preferable arrangement is to install two intake fans in the front of the case and two exhaust fans in the rear to blow hot air out of the case. The most important consideration when selecting a fan is its air throughput. The higher the CFM, the more effective the fan will be in moving air through your system. These fancy fans are handy in the event your motherboard doesn't provide for hardware monitoring.
Ball bearing fans last up to three times longer than sleeve bearing fans, and cost about the same.
Selecting a fan for the lower front of your case should be limited only by the availability of pre-existing ventilation holes. Select the largest square fan with the highest CFM that you can locate. Sizes range from 20mm to 120mm. Install the fan with the label facing in toward the interior of the system. Such fans come in dual or triple-fan configurations, and should be purchased according to their CFM ratings rather than price.