For comparison, an old Dell business desktop gave us 41dBa at idle, a PlayStation 3 gave us 42dBa and an original Xbox 360 (not Elite) gave us 47.8dbA. It's said that a 10dB gain is perceived by human ears as a doubling of volume, which seems about right -- the Xbox sounded far louder than our HTPC.
ENGADGET: How-to: Build a multi-talented HTPC for (roughly) $1,000