Best airflow can be obtained by sucking air in at the bottom of the case (front or back), and blowing it out of the case at the top. As air gets hotter, it rises up, so the hot air needs to exit the case at the top. Also in most configurations, the cpu (which is one of the hottest parts that need cooling) is at the top of the case. With a fan at the top blowing outwards, that hot air produced by the cpu area will get blown out with the rest. Look at these representations of how airflow should be directed : Depending on the amount of fans, always try to get air in at the bottom front or back, and let it blow out at the top back. Bottom front is better than back, because then your harddrives will be cooled down too in the process. Cabling is also a factor to take into consideration. It needs to be tightly tucked away along the sides to maximize airflow. This is a pic of my own case. It's an Antec 900. It has 2 x 120mm fans in front, blowing air in and cooling my drives. The air blown in from the front gets blown towards the top with the help of the psu fan, thus cooling graphics and ram. The air gets blown out at the top by a 120mm fan in the back, and a 300mm fan on top of the case. CPU cooler also blows air towards the top, straight out the case. Cables are tucked away against the backpanel and along the sides as much as possible. As you see the motherboard area is unrestricted, allowing maximum airflow : My temps, to prove the effect of proper airflow : Hope this clarifies a lot about managing airflow with fans...
lol can't u see the fan of the psu pointing upwards in the case ? the bottom of the psu is closed of you know...only one direction it can blow.