Personally I don't care for top or bottom fans.
Top fans too easily get things dropped into them and get in the way of setting anything up there.
Bottom fans tend to suck dust bunnies off the desk and blow them into the case.
I also don't like negative or neutral ventilation plans.
Positive ventilation (slightly pressurizing the case) helps keep crap out of all the cracks in optical drives, card readers and such.
Positive ventilation with filters keeps everything pretty clean inside as well as in the cracks.
For me filtered front or side fans all blowing -in- is the way to go.
The only exhaust fan is the one in the PSU. (These are typically around 40 cfm regardless of diameter.)
Vent holes should be opened or covered until there is a steady gentle draft blowing out from inside the case.
Two well placed 92-120mm fans is usually plenty of flow even for a hot system so long as you've adjusted the air flow correctly.
Three of course can be quieter and achieve the same thing depending on the fans.
Too many unfilled holes (especially near a fan) will cause air to be short cycled which means the air is just going in a circle through the fan and holes and you aren't really getting full flow -through- the case.
A tower case is approximately 1.5 cubic feet so 2 or 3 properly set up inlet fans (no short cycling) providing 90 cfm total (after filter drag) will change 100% of the case air once per second.
That is actually more than is needed.
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GRUMPY OLD FART - On Hiatus, sort'a
Mann-Made Global Warming. - We should be more concerned about the Intellectual Climate.
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