I'm looking into one as well.  I have the TV tuner card I'll be using, a Fusion HDTV7 RT Gold from Dvico, nice card, supports un-encrypted QAM digital channels (although my carrier encrypts all but the audio QAM channels).  I'll probably end up upgrading my rig and then using the bits and pieces from it for the HTPC.  It'd end up having a C2D E6300, 3GB of DDR2 OCZ Gold, 3x500GB in a RAID5, XFX 7950GT XXX (silent cooling FTW, no fan on it), built-in sound, and I'll get a nice small HTPC case to hold it all.
Not sure if I'll go *nix or MCE on it, or something like SageTV.