If you haven't see bitSync's "Silence is Golden..." post referenced in the second post in this thread, you should - very impressive. But bitSync's skill set is quite beyond mine so I did something a little different.
As described in his excellent and well documented post, I removed the CPU fan, which in my instance was very loud. I replaced the hard drive with a compact flash. But rather than replace the power supply I replaced only the power supply fan. There are many quiet fans out there but most of them move too little air. The fan in my d8b's power supply was an AD0812HB which, according to specs, is rated at 39.6 CFM and 34.4 dB/A. I replaced it with a Silenx EFX-08-15 Effizio which is rated at 32 CFM and 15dB/A. I left the top off the d8b's CPU and placed it in a dedicated rack near the top where air could convect and yet objects wouldn't fall in the unit.
After 2 months of almost daily use, sometimes as a d8b and other times with the proBox, I've had no problems. When I first turn the d8b on I really can't hear the fan. After several hours of use, presumably due to warming in the unit, the fan runs faster and I can be aware of it, but unless I think about it, I am not aware of it. I have been able to mic in the control room without bothersome fan noise.
Certainly bitSync's fanless mod is preferred, but this has suited me quite well.