One has to consider airflow within the case.
Try to make it move from front to back, from bottom to top.
Ideally one should have the same number blowing in as are blowing out.
When you have an odd number make the extra one blow in as long as that air can get out someplace.
I have a 120 in front, a 140 on the side, an intake snorkel vent over the CPU (a downdraft heatsink w/ heat pipes still works just fine thank you very much) and a 120 plus the PSU and GPU fans exiting in the rear. Inside the case is the CPU fan and an extra helper fan I glued onto my GeForce.
I taped up all the holes and cracks in the sheet metal and made a baffle from a plastic bottle to keep the wires off of the mother board. When people open my case the freak out because of all the wires. I have no wire management in my case so I made my own. The baffle presses all the wire against the side of the case, leaving the center open for airflow.
One can get cheapo fans, but as UGLY as they are I like the NOCTUA fans I am using now. Very quiet and cool. I also HIGHLY recommend getting washable foam FILTERS for all intake fans. Since I got the filters, the inside of my PC gathers NO DUST. I still clean it out, but there is hardly anything there to clean.