You have to be careful with "Digital Cable" you would not want Digital Cable but instead HDTV Digital Cable.
They are NOT the same thing.
Digital Cable is "standard TV (NTSC)" sent digitally. The cable company does this so they can compress the regular channels and provide more channels. Therefore the end result is regular TV.
HDTV Digital Cable has the HDTV (ATCS) channels.
If you go with a PCI HDTV Tuner then it can only receive & (assuming your graphics card & display can handle it) display the Over The Air (OTA) HDTV channels. Usually (depending upon where you live) receive the digital signal for the big 4 broadcasters.
ABC, NBC, CBS, FOX (sometimes PBS & UPN)
In addition antenna direction plays a critical role in either receiving or not receiving a channel. For example: The ABC & NBC HDTV broadcast towers are north of my location & the CBS & FOX HDTV broadcast towers are south of my location then (with 1 antenna) I can either receive ABC & NBC or CBS & FOX but not all four.
Also (unlike OTA NTSC) with OTA HDTV you either have a signal or you dont their is no snow/static of a partial picture.
http://www.hdtvpub.com/
This link has some information on OTA HDTV channels, I remember seeing another website that actually could show where the broadcast towers were in you area by zip code but I can't seem to find it.
HERE it is:
http://www.antennaweb.org/aw/welcome.aspx