“Heatsinks aren`t warm even when running hot” indicates poor thermal transfer to the heatsink. As for the power supply, it does have its own cooling fan (typically 80mm or dual 80mm) drawing hot air off of the CPUs and using that hot air to cool the PSU. Therefore, the top venting fan is unnecessary and only serves to cool your optical drives. To add to the problems with a side intake and top exhaust fan, the airflow must turn 90 degrees almost instantaneously and creates swirls and turbulance as it does so, making the air stagnate (think of the vacuum behind a car for an example since it’s the same effect), whereas a traditional front intake rear exhaust (even adding an additional top 92 or 120 mm) is much more efficient as the air turns more gradually, allowing for laminar flow through the majority of the case except where it hits components with heatsinks on them (or your RAM too). To add a side mounted intake fan to this, it should be mounted low, over the cards, in a naturally turbulant area of the case with little airflow, to add additional airflow to there and allow the turbulance to occur in a naturally turbulant area anyway, and then it’s drawn into the primarily laminar flow into the rest of the case.
pick up a fluid dynamics textbook and look up turbulant versus laminar flow.
And last I checked, PSU fans were at the extreme top of the case and drew hot air out of the top of the case. I don’t know about you, but I ran a fluid flow analysis on my system before I completed assembly to optimize fan and cable placement and found that if I were to place a side mounted and/or top mounted fan in my current setup it would actually cause air to stagnate in the turbulance and superheat, with some parts reaching over 200F.