There's not much to it, if you're going for quiet, small or low power, make sure to offset the slower CPU with a good GPU to offload that processing onto (nvidia probably being the best currently for HD video, make sure there are at least 16 cuda cores). Otherwise, any decent dual or more core system (preferably with a decent 16+ cuda core nvidia graphics card [or equivalent ATI], if you want to free up the CPU for other tasks).
For my main projector computer, I have a pretty simple setup, a 6000+ X2 (dual-core, 3GHz per core) processor (enough to handle 1080p Hi10p, if the player is set up for multi-threaded decoding), and a fanless nvidia G210 16-cuda core GPU (enough to handle all my non Hi10p HD needs, so long as I'm not running a desktop environment that is overly 3D hardware intensive). Audio also goes out over HDMI on the video card to the receiver, so that rules out having to get a crazy sound card anymore. I run XBMC to access all my media from my file server, and that's about all there is to it...
In the kitchen for the wife, I have a Zotac Intel Atom computer with an nvidia ION GPU, VESA mounted to the back of a monitor, and that works well also.
Heck, I have a couple of Asus N10J mini-laptops that can do 1080p video playback and bitstream digital audio over HDMI very well, and that's quite a diminutive system...
Really, the sky's the limit these days on what can be an HTPC.