My own experience tells me that it is all in the parts selection.
start with a case that has really good airflow characteristics, or modify one until it does. Use multiple quiet fans,(or run them at reduced voltage), PSU with temp controlled fan, and a quiet HDD like a WD or IBM. Sacrifice a few Mhz off that radical overclock to use a quiet fan on a large cpu heatsink.
Various sound absorbing materials are useful only as the finishing touch to already quiet systems. Knock 3Dba off of 30, you can tell a difference. Knock 3Dba off of 60, it's still loud as hell.....
The best way to kill noise is to not make it in the first place. It's easy to fall for the idea that more cooling is always better. There is a point of diminished returns, many overclocked fan freak systems are well beyond it. Your ears pay the price for that overindulgence.