Feel free to make ANY suggestions. I would absolutely appreciate it. (esp the tuner+GPU)
Whats the difference between using the comcast cable box or a TV tuner? Other than not paying for one... It wouldnt be a huge deal to switch inputs. Are there any other advantages worth paying $150 for?
Well, the TV Tuner might be a good add-on for your HTPC. It's not essential though because you have the TV right there. You could probably save yourself 100 bucks on a good tuner by just switching inputs on your TV whenever you felt like watching TV. I would only suggest getting a tuner if you don't already have a TV (monitor instead). If you decide on getting one anyways, choose Hauppauge. They have a reputation for getting the job done.
It really depends on what channel package you subscribe to. Pretty much anything above basic cable is going to include channels that are not sent in ClearQAM (unencrypted). While Comcast's set top box (STB) will be able to decrypt these channels, a TV Tuner card will not.
If your STB does not include DVR capabilities, this is something most (if not all) TV Tuners cards can do.