I do exactly this from my DirecTiVo box. I run the digital out from the TiVo into my receiver, and the digital out from the receiver goes to an external USB sound card (my, now discontinued, Onkyo SE-U55). From there, you just punch "record", watch your TV show, and punch stop when done. Cut, splice, compress, tag.

I've used this to rip music from several "live" shows. My favorite source for good music is Sessions at West 54th. It only ran for three seasons on PBS, but Trio is rerunning it in syndication. Other sometimes useful shows A&E's Breakfast with the Arts and Bravo's Musicians. Trio on Tour occasionally runs bands I like as well.

Any other suggestions?