Depends on what components you choose. My MCE Rig is as follows:
5 DVB-t tuners (4 Hauppauge Nova-T PCI + 1 Avermedia USB2 A800)
Accent HT-400 Case + Acoustipack silencing kit + Seasonic S12-500 PSU
Asus P4C800e-Deluxe + 2 x 512Mb Corsair RAM
P4 2.8GHz (Northwood o/c'd to 3.2GHz) + Thermalright XP-120
nVidia 6600GT (fanless) + nVidia Decoder
1 x Samsung 40Gb Notebook drive (OS - MCE2005)
2 x Samsung 250Gb in RAID 0 array (TV recordings)
To all the people that have witnessed it, my system is silent (even passes the girlfriend test). If I was being really picky I can just about hear the soft seeks of my hard-drives but these are drowned out at even the lowest of speaker volumes.
The Samsung Spinpoints are class leaders in terms of noise. You can reduce it even more by setting the AAM of the drive's bios to "Quiet". I'm impressed with a new HTPC case from Zalman (HD160). It has some classic mods which are commonly used by the silent PC crowd (i.e. rubber mountings for hard-drive cages, air-duct for PSU, optional air vent for CPU). It's large size ensures that you can use ATX powersupply (Seasonic S12 have no equals in the efficiency and quiet stakes), ATX or mATX motherboards, and full sized CPU coolers. Although the price might seem overly expensive (when compared to other HTPC cases) it does include the VFD (which is normally an extra £50) and also the MCE Remote control + IR receiver (the receiver is integrated into the case) which normally costs another £30.
If I had that case, I would use take advantage of the PSU air duct, have something like a Thermalight XP120 with the fan
pulling up into a duct for the CPU vent. That way you've taken two heat generating components out of the system. The two 80mm case fans would be enough to exhaust the heat generated by the rest of the components in the case.
The only downside is the noise they make. Extenders might eventually reach these shores and look like they'll be affordable
http://www.engadget.com/2006/03/15/p...-on-the-cheap/. Maybe we'll see these come out when Vista is released?