The only thing special I added to ours was an HD Video card with an HDMI output. I got
this one. Good price at ~$50 (I actually got it on sale for $35), no moving fan so no noise issues, and unless you plan on doing any serious gaming, perfectly good picture quality for viewing video content.
You can even set it up to be dual-monitor so you can watch TV and do other stuff at the same time.
Mind you, we're not using the PC as a DVR, sticking with TiVo for that. Can't help you much there. MythTV has a good rep, but it's somewhat of a pain to get running on Windows. SnapStream is a Windows option, no idea if it's any good. Nor do I know how well any of those options work (or if at all) with cable box vs. cablecard.