SnortonNorton's build, day one.

[shardian]

Having an in/out fan arrangement creates a more laminar flow of air. If you just have air holes for an output, your air flow will be very turbulent, and some air will circulate around inside the box longer than you want. The longer air circulates around a heat source, the hotter it gets. It may only cool the box a few extra degrees over a hole arrangement, but cooler is always better.

[scanboy]

I believe stocker was talking about air holes as input, so you just need one fan to suck out the hot air. If your holes placement were correct, the air flow will cool whatever that you wanted to cool. If fact, if you have a higher flow fan blowing air insdie and a lower flow fan blowing air out, this will also create air turbulance inside.