You are referring to the case fans. Well i used one in front of my case and one in the back. If you have a fan speed control you could even control both fans to balance the cooling.
The front is mainly used to cool the HDD if the bays are in the front. If you had two or more HDD in your system I would use two fan in front and one in the back. Look at the motherboard to get an idea of the computer brand use a flashlight it helps to see the print on the motherboard.