You might want to modify the case. The drilled holes are to small and block air flow. Thats why so many people cut out big holes and put grills in the case. By default there is only room for 3 80mm case fans (two top rear and one bottom front).
Heat rises so it usually good the exhaust the heat at the top of the case.
With modification it's possible to fit a 120mm case fan in the top rear. Bigger case fans usually pump more air per decibel of niose produced.
The advantage two having simular amount of air blowing in as been exhausted is that you don't create a vacuum and have air been sucked in through the floppy drive, which would clog it up with dust.
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