Slappy,
While I have TWC, I strongly suspect HD access will be the same with Comcast. Comcast should have some type of digital cable box with HD capability. TWC makes you rent theirs. It does the decoding. You'll need an HD ready set for HD and for progressive scan dvd. Most people use component video inputs for HD but RGB can be used if your cable box has RGB outputs. For progressive scan you must use component video.