I recently redid my HTPC for HD OTA, based around a gaming laptop
with HDMI out for low power consumption. The laptop has an ATI HD3470 in it, so it offloads most of the video playback to the GPU, I get about 15% CPU while watching 1080i, I think the 15% is only because of Vista Media Center insisting on using its DVR-MS format. The idea is that laptop is used 24/7 for watching/recoding shows, and torrenting in the background. If my tests go well, I'll cancel my Rogers cable and PVR.
I have a separate PC w/LG combo drive + ATI 2600Pro for HD disk playback, but hope that I'll be able to migrate that functionality to the laptop as well (I'll get an external case for the LG drive, then hook it to the laptop) in the near future.
Definitely more expensive than a standard PC HTPC, but also lower powered, and silent right from the start, no extra/quiet fans, special cases, coolers, etc to deal with, and I'll 'recycle' it back to my desktop in a year or 2.
here's some details from my first attempt 2 years ago, and I'll be updating it with my latest adventures soon: