See HTPCnews.com. This site also has some excellent forums.
These threads may be useful:
In 2004 Anandtech did a two-part article, Building a Linux PVR Part I - MythTV Setup and Install and Building a Linux PVR, Part 2: Microsoft's MCE 2004.
These softwares may be used for your HTPC:
The MediaPortal wiki lists compatible cards:
Lightweight Ubuntu distros - Xubuntu, Ubuntu Lite, and Fluxbuntu.
Linux specific HDTV tuner cards: http://www.pchdtv.com/
A useful site is http://www.linuxtv.org/
Tuners that appear to be well supported under linux:
There is a HDTV version of the Kworld… DVB-T220