As much as I am a computer nut I would have to say that a Tivo-HD with a lifetime subscription is a more cost effective solution.
PC tuners are fine but there are very few (like one or two) that have cable-card compatibility, and those are quite expensive. So this limits your viewing to only digital cable channels that are sent in the clear. On my local Comcast system only the local networks (ABC, NBC, CBS, FOX) are sent in the clear. So I can only really get six or eight digital channels on my PC. I still get the analog extended basic channels, but analog of course.
But if you do indeed chose to go this route I would recommend the Hauppage (I think that's how it's spelled) line of cards as they seem to be the most compatible and have MPEG hardware encoding on-board.
Oh yeah the dual tuner thing is a must have you can record one channel while watching another, or record two channels while watching a previously recorded program. I know I thought we would never need it, but it's amazing how often we are using both tuners in our Tivo.