Just remember the fan is there to stir and mix the air in that bottom corner dead spot and that’s all. This works well as long as you don’t over do it to the point that it will interfere with the vga cooling fan and the airflow going through the heat sink. Adding a side fan blowing cool air into this bottom corner helps 'stir' the air so heat doesn’t get trapped there and it can rise up and be exhausted by the rear fan. This is why they have fans now that exhaust hot air directly out the back of the case.

VGA - the video card area tends to be a dead spot for airflow. To reduce the interference from side fans most cases use air ducts to direct the cool air right into the CPU fan so it will not interfere with airflow through the case so side fans should never be used without air ducts to prevent the whirlpooling of the case airflow. Side fans are great cooling aids for direct cooling but reek havoc on proper airflow thru the case because they just cause the air to flow in a circular pattern and the heat has trouble leaving the case so it makes one spot cooler while making everything else in the case constantly stay warmer so while side fans can be helpful, don’t go overboard with the cfms. It may improve cooling in one spot and allow a 'dead spot' for airflow somewhere else. Side fans - side fans can and do interfere with this 'S' flow of air so keep this in mind if you’re thinking of a high cfm side fan. The air is drawn in from the coolest point (bottom front) and moves in a lazy 'S' upwards to flow across the motherboard and CPU and then carry the heat from the case out the rear exhaust fans (top rear and psu fans). The reason they do it this way is it takes best advantage of rules 1 & 2 above. The most common setup for cases is to intake air from the bottom front and exhausted out the top and back. The airflow can be directed into the case from a few different ways as long as you remember rules 1 & 2 above. Rule #2 - Heat naturally rises so to remove heat the easiest and most efficient way it is best to draw air in from the cases lowest points and exhaust it from the cases highest points. another thing to note is noise, a fan is always louder when mounted onto some type of a stamped metal grill then with it just mounted over an open hole, sometimes its way louder. Redrill all those small holes in the vents at the back of the case with a 3/16' drill bit to increase air from that spot and if you have the larger hexagon shaped holes that are already larger then it should be ok the way it is. the newer and larger openings and fan grill designed cutouts are ok but if you aren't sure then test them by feeling the airflow with the fan outside the case then in the case and if its unrestricted you shouldn't be able to tell the difference. cutting out the opening for the fan can increase airflow by as much as 70% more then the fan trying to get air to pass thru the holes. Also always cut out a hole where you mount a fan, never mount it behind those predrilled 'cheese grater' holes for fans or you won’t get any kind of good airflow.

Rule #1 - Airflow is best if it can flow in one direction only and it is always better off (stays stronger) flowing in a straight line of sight direction if possible.

Pc case airflow diagram