I just put together a Shuttle XPC for my mother.
I can attest to its incredibly low SPL, virtually silent. The vidcard runs a bit hot but is otherwise a nice little unit, all for under $700 Cdn.
The parts are more mid range now (Athlon XP 2500+, Socket 754) but would be more than enough to run as a HTPC. I would probably change out the video card for something a bit more beefy though. I would also go with a separate sound card.

"Those who preach the myths of audio are ignorant of truth."