I can record two HD ATSC channels at the same time and watch a blu-ray disc with my Celeron E3200 and 2GB of RAM. Recording HDTV does not require many resources at all.
I'd love it if the next Xbox came with a TV tuner built-in. Cablecard would be an extra bonus. But I don't think that will happen. Everyone seems poised to move to IPTV. Comcast has a deal on the Xbox to stream channels. It's much easier to implement IPTV than it is to send out channels to subscribers' homes and support decrypting boxes with slow, confusing menu systems.