I did the CableCARD hack, and it worked for me. The tuner was $195 on ebay...I got the external one.
There is no difference between the internal and external...they are both USB.
The issue with CableCARDs is they will not do On-Demand or any channel that uses SDV (Switched Digital Video), but they will get QAM. I know for me, I get all the channels I care about...the multiple secondary iterations of HBO and Showtime are SDV but the main channels aren't so I'm fine. You'll want to check with Comcast about what is SDV and what is not...and that assumes whoever you talk to there will know...and that is not likely. And, yes, I get HD and SD channels.
With Comcast, recurring costs for a CableCARD will be <$2. I think the cable companies are being a bit careful about what they move to SDV and what they keep on QAM. CableCARD users tend to be pretty sophisticated and vocal...and there's only so much they will tolerate before they call the regulators...moving every channel to SDV would do that. I know that the cable company here tried that and there were newspaper articles about it.
Another very-low-cost option is to install one or many QAM capable tuners...this will let you record the major network HD channels for no recurring monthly cost. Major network meaning anything that is broadcast over the air.