Have you checked for quiet cases from the likes of quietpc.com?
My MythTV machine uses a 'hifi' looking media centre case (see
http://www.cluboc.net/reviews/cases/antec/fusion_black/front.jpg). Airflow is managed well, fans run slowly, the remaining noise being the hard disk(s) occasionally chattering. The HDs are mounted on rubber mounts (included in the case) so the only way I think I could baffle them further would be to switch to SSDs.
I can highly recommend the above case both for sound insulation (the fans are supplied with a 'low speed switch' on their power leads) and thermal management (PSU has separate compartment, plastic airflow ducts to exhaust/direct air over CPU heatsinks included).
Decent case, quality PSU, fanless GFX card, low power AMD (or similar) power efficient fanless CPU... job done
