If you have a 1080p TV your analog channels are probably going to look like crap. A good TV shows how bad the video that goes out on most cable or air stations really is. I just got a 46" Sharp Quattron as an upgrade from a 32" Samsung 720p
and I was stunned at how much a difference it made.
I'm not sure with Comcast, but, even with just basic cable you should be getting some digital programming - we do on Time Warner - most of it's either NBC, ABC and public broadcasting (our tax dollars at work).
It is very doubtful that a PC tuner will make the channels look better.
As far as I have seen, the purpose-built receiver hardware in the TV set works a bit more smoothly than any of the PC tuners (I have a Haupage 2250 pci-e on this one).
The whole purpose of the Tuner card is really to bootstrap using the PC monitor as a TV - not to replace the TV's tuner - and you are basically just adding a bunch more electronic plumbing to the mix (PLUS DEALING WITH WINDOWS!!) to use the PC tuner verses the TV's internal one.
However, there should be an option in your TV menu to adjust the picture aspect on the fly to get rid of the black edges (use the remote to check thru the TV's menu or there may be a button - its usually called something like "view mode" - that allows you to cycle thru a number of screen aspect modes.
Also, be sure to check any "digital channel scan" setup options available on your TV. You may find you have more digital input than you think.