Ideally, you would have a push-pull system employing two fans. Push the cold in low and pull the hot out high. If I was only going to add one fan, I'd opt for pushing in since the power supply does provide a small amount of pull out. Put the fan as far away from the power supply as possible so that the incoming, hopefully cooler air, travels over as much circuitry as possible before exiting. A lot of mini, mid and full size tower cases already have cutouts in place for one or two assist fans, though often, in the interest of appearance, the intake (lower) cutout is obscured by the front plastic escutcheon. Many people either drill more small holes or one big hole in their escutcheon to yield the least resistance to air flow as possible. Opening up my intake path dropped my case temp 5 degrees. That can be significant is your HSF is working hard to make ends meet.
Hoot
