Airflow: Horizontally-Mounted Tower Cooler
Horizontal Installation of a Tower Cooler
Let's go back to AMD's Socket AM3 and take a look at a horizontal mounting of the cooler. What initially seems to be a disadvantage can actually turn out to be an asset. Remember the stack effect? If warm air rises, why not take advantage of that fact? For horizontal mounting, you need a case with ventilation up top to make this work.
We also make use of an additional exhaust in the back, since many tower coolers blow some air on the surrounding components (voltage regulators, for example), and this "scattered" exhaust also needs to be vented. Horizontal installation is also possible with a top-mounted power supply:
This scenario really makes a top-mounted power supply's disadvantages obvious, however, so we definitely advise against blowing all of your processor's heated air up into the PSU. Really, there are so many better solutions.
If you decide to go this route anyway, make sure to at least include a rear exhaust vent:
Bottom ventilation helps create extra cool airflow: