I presume if it's vented properly that wouldn't be an issue?

As a Mac guy, I have followed the heat management issues that have been so important, they have dictated corporate policy.  The reason that Apple dropped IBM for Intel is that IBM either could or would not come up with a more energy-efficient chip.  The G5s had 11 fans to manage the heat in a dualie,  When Apple switched to Intel, the same case that held the G5 could now hold 16 gigs of physical RAM, and 3 Terabytes of hard disc storage because there are fewer fans and dramatically smaller heat sinks.

My G4 has a pretty noisy fan but it is nothing compared to the late G4s that were called "wind tunnels" by their critics.  Apple actually put a version of my CPU in a laptop.  But I have three hard drives and I assume heat is probably a bad thing for electrical componets (I still remember tubes) so I am being careful.  I'll probably over-engineer something, if the past is a guide.

"Remember the I35W bridge--who needs terrorists when there are Republicans"

One of the best source of information on the topic is SPCR:

http://www.silentpcreview.com/

And check the forums they're quite friendly:

http://www.silentpcreview.com/forums/

To get an efficient PC first thing you'll need an efficient power supply (PSU) and that means building your own PC since no brand name PC uses a really good PSU. By good here means high power efficiency meaning both lower elctrivity bill and less useless heat to evacuate hence more silent PC so every percent count :).