With comcast, the Analog and QAM tuners will get you all the analog channels you pay for (basic is like 20-30 channels, expanded is up to about 100 or so in my area) and also the unencrypted digital stations, which includes some music channels, other peoples on demand and Fox, CBS, ABC, NBC, PBS and CW/WB/UPN
If you are paying for HBO or Showtime or anything like that you wont be able to tune it at all most likely.
Getting the HD box lets you tune whatever extra channels extra you are getting. If you have expanded basic, you will have access to ESPN HD and all the other non-movie HD stations available. If you are paying for HBO/Showtime you will also get HBO/Showtime HD.
So I guess the question is, what cable service are you paying for currently?
Also IMO the HD Cable box is a waste of money, since the upgrade to an HD-DVR isnt much more. So I would suggest no box or go DVR, but that is me.