I'd investigate motherboards first. My one has DVI, HDMI, S/PDIF, etc all on board. (Isn't HDMI just DVI & S/PDIF rolled in to one connector?) Some boards have Nvidia 'PureVideo' graphics chips on board too which is good if you're going to run a Linux based PVR like MythTV or Freevo.
For those on a budget, this might be a good start
eBay link.
To watch HDTV takes a decent CPU if you don't have hardware decoders on the video card, hence Nvidia 'PureVideo' being a good thing. My old P4-2.4GHz systems system struggles with HDTV.
