My answer will make you all cringe.
I'd been using a C2D mini for years. I liked that it was small, quiet, and included an optical audio port. I ran Windows Media Center as the UI, it was too slow under OSX with HD video. The same hardware had no trouble with HD playback under Windows at all (using Core AVC decoder).
I finally upgraded it to a newer mini, mostly so I could mess around with a Wii emulator. I might give OSX-based HTPCing a try again.--
AT&T U-Hearse - RIP Unlimited Internet 1995-2011