The key to making this rig quiet is my choice of motherboard, hard drives, heatsink and video card. The motherboard is an Intel D865 PERLK which features what Intel calls "precision cooling technology". This basically means that the 3 fan headers on the motherboard features dynamic fan speed control based on built-in temperature sensors. The nice thing about this cooling technology is that it is BIOS controlled so it works both in Linux and XP. The fans basically rotate at much lower speeds (<1500 rpm) when the PC is not being stressed, making for quiet operation. They spin up gradually as the Chassis temps go up (up to 2000+ rpm when doing CPU intensive stuff like compiling big apps in Linux or playing games in XP).
I attached the rear PSU fan, top blowhole fan, and rear fan to these fan headers. The 2 fans on the front are controlled by Lian Li's fan control switch which are defaulted to the lowest settings. Here are the temp and fan speed readings from a screenshot of the Linux monitoring utility GKrellm (taken at an ambient temp of 26.5 C):
This PC has 2 Seagate Barracuda hard drives (one dedicated to Linux and one dedicated to XP) which are pretty quiet even though I did not use any damping. I used the Zalman 7000 HSF which does a pretty good job of cooling the CPU. The video card is an MSI FX5900XT featuring an oversized heatsink with an oversized fan that, like the Zalman HSF, spins at lower rpms.