As a Comcast/Xfinity customer, the card you want is the Ceton InfiniTV4/InfiniTV6 TV Tuner card (and a cablecard from Comcast). This removes the need for a set top box from Comcast. Otherwise, look for a TV Tuner card with either NTSC or ClearQAM tuners.
Thanks Wolf - I'll look into the TV Tuner card - any suggestions for something inexpensive? The DTA is only $1.99/month from Comcast. Not sure what they charge for their cablecard, but I'll check that too.