If it is placed under your TV then why does it have to be quiet? I would think the TV sound system would easily knock out any sounds from such a unit even air cooled. I can't hear my computer over my TV and it is currently air cooled and it is an old TV with stock speakers.
Also does a HTPC really have to have so much CPU power? Why not use one of the VIA/Cyrix chips that can be cooled with a heatsink? 7 watts at 1000mhz should easily run that thing.
http://www.via.com.tw/en/products/processors/eden-n/
But anyway whatever. Good luck.