you're far better off showing us your case, because using a side window fan can seriously screw up your airflow and i cant tell till i see a photo.
in the bottom front and out the back top is by far the best set up, but even this can vary
a side fan blowing in can cause a whirlpool vortex cutting off the intake from the exhaust, thus forcing the case to heat up, there was an expert here on this forum a few years ago that studied all of this, plus tried different set ups
Generally a side fan is used as an intake.
But on my own case, i tried it as an intake, exhaust and with no fan at all.
Didnt really make much difference in gpu or cpu temps while stress testing. +/- 1.C, but that was just in this case, yours may be different.
you have a Antec 900, my kids have the first one, i just blocked the side fan hole on that case, it was tiny anyway, 120mm