You have a lot of small MD you can buy on the market.
If you want it prebuild, you can use any ION based PC (I am planing to upgrade to ASUS EB1501, which has DVD drive on it). If not, I would also recomend any ION based MOBO. MD doesn't need HD as it gets anything it needs from the network.
I build my new Hybrid last weekend, mainly from old parts and the new ones where bought "low-noise-wise"
New Asus P5QL-EPU, check this: http://forum.linuxmce.org/index.php?topic=9637.0
Old 2x1gb ram
Old HD for system (160gb)
New HD for media on its way
New noctua nh-u12p fan (sure it is not cheap, but worth every peny, now processor temp is 32º, and barely audible)
New GT220. check this link for HDMI audio: http://wiki.linuxmce.com/index.php/NVidia_GT220_HDMI_sound
old 450w PSU (nox urano)
Used it with UI2 W/Alpha and up to now, I saw no tearing.
With the links I gave you, you have the Hybrid working almost out of the box.