I doubt it's worth the effort to make it silent.
The cooling fan on a p3 is very quiet. It's all those case fans that create the noise.
The powersupply has a built-in fan, too.
Try it out using just the PS fan and the stock CPU cooler. See if that is quiet enough for you. I bet you can still hear the hard drive. (good luck making that quieter)
P4 firstname.lastname@example.org dual R134A phase change chillers with 600watt thermoelectric array. Capable of -70F
Nixie tube numeric display control panel and pneumatic motherboard tray.