Morbs, classical way of putting fans.

One in front soaks cold in.
one in the rear that pulls hot out.

==> PC ==>

Air should move.

If you plan to place just one than Stone is absolutely right, place vent to take the hot air out of PC. Ahh, and don't forget about dust. There is great difference if you PC case is on th floor (70% of all dust) on the table, etc. If you have vent to soak air in, think about filter. That's my suggestions.

here take a look mate.

"Endure. In enduring, grow strong." -Githzerai adage.