Well the curious thing is Microsoft envisioned the PC to be the center of the home theater experience, which was what their media-centric playback software was designed for. They believed people would turn their office rooms into mini-home theaters with PC speakers and watch TV on their computer monitors.
Instead people started moving their computers into the living room instead and used it as a "hub" for their home theater.
Today the (most) basic HTPC is a net top with 1080p playback ability, maybe run some emulators.
A more advanced HTPC would be a one source solution for your entire home theater...tivo functionality with a tv tuner, IR receiver/remote for Windows Media Center.
Still, HTPCs are defined by generally small form factor and quiet operation (home theater components are typically are silent).
Snowman's setup is an HTPC albeit uncommon, since it's not specialized for HTPC use per-se.. I'm also planning a gaming HTPC, however, I don't have a convenient closet to lock it into, and with a high end video card it will probably be fairly loud, counter to what an HTPC really is for--a simplified, all-in-one box for home theater needs instead of running a billion receiver/pre-pro/pro/pre-amp/insert random receiver box...setup.
It's really a painful choice between noise/processing power...you could get a giant metal case with heavy dampening but then your parts will probably cook unless you underclock the hell out of everything...