(I have a combination energy recovery ventilator/HEPA filter that has has a restriction filter on its exhaust for this reason. Else it would just suck crappy air in from the basement instead of fresh air through the filter.)
If you have to choose one, choose an intake fan for this reason. Best if you can leave another window slightly ajar.
We also have its accompanying HEPA unit: http://www.purifresh.com/cleanroom.html They can be linked (as http://www.purifresh.com/freshairsystem.html) or used separately. (Apparently this is out-of-stock and being redesigned.)
The highest performing PC fans create about 10mmH2O pressure while typically you can expect about 1mmH2O pressure compared to 10mH2O of the atmosphere. This translates to densening of the atmosphere around components by 0.1% or 0.01%, respectively.
Correct placement and design is by orders of magnitude more important than the increased efficiency due to higher density of air.
You could see much more effect by making your air more humid (humid air has higher heat capacity), which will be also much more pronuncable than the increased density due to pressure.
I think it is because you can measure it yourself with just a tube and water- open end manometer.