First off let me say the best to to kill a computer is to over heat it.
Intel chassis are designed and a lot of effort goes into thermal evaluations of the board and chassis components to keep everything in a safe operational thermal range.
That said, there is nothing to keep you from doing the same type of thermal evaluation.
Best general rule is that the free air ambient temperature anywhere on the mother board should not exceed 55deg C and if you it down ~ 40degC the board will be much happier.
Fans are the easy way to do this, since they just move the room temp air (25 degC ) throught the chassis to keep things cool.
Most chassis are rated for room temp up to 35 deg C operation to allow the internal temperature to remain below the 55 deg C mark.
The other piece of the puzzle for computer and chassis designers is FCC emissions regulations and restrictions.
That big metal box which traps all the heat in also traps all the radio noise emissions.
Use of a lower power (30 w) CPU and a oversize passive heat sink will help to keep the processor cool, but you need to provide some way for the heat to get out of the box.
Thanks for the reply and concern. I definitely don't want to kill anything.
The case I'm intending to use is aimed at media PCs (I might repurpose this setup towards that in the future) and has been around for at least five years. It has fins on the sides to dissipate heat and from them heatpipes are routed to the CPU (or GPU) heatsink. With that heatsink, a quad-core i5 will not go above 55 degC at full load (<25 degC in room), so ambient temperature around the board should be much below that. I intend to use an L3406 (or at worst a Clarkdale i5 or i3), and as the TDP is almost a third of that for such an i5, I'm expecting much lower temperatures. For backup there will be a sensor controlled 92 mm fan with customizable temperature threshold, not sure of the CFM rating though. The setup I'm looking at is linked below. It's a very, very nice case.
There shouldn't be any trouble with the board not powering up at all, as long as the temperatures are safe?