Why would you underclock a C2D E6300? It's already slow enough as it is...
If you're looking for silence, you can get a cheap low RPM aftermarket HSF cooler, or get a large cooler such as the Mugen 2 and use it as a passive cooler.
You can even overclock with better coolers. (The Mugen2 can passively cool a Q6600 quad core)
I am building a music machine, cMP2. see www.audioasylum.com in the audio pc forum. Two customers want this "Recipe" using a Gigabyte GA-EG 45M-UD2H mainboard. silent pc's are in big demand for audio rigs. Line in to high end amps with usb dac's. Thanks for your reply,Alan