To be honest, even I don't fully understand the way Comcast bills for services when it gets down to service fees, licensing fees, and how they tie into their monthly charges.
Unfortunately, I do not think its wise to purchase your own converter as once it's installed, it still need to be activated by Comcast. If the converter you purchase does not meet Comcast specifications, they can simply refuse to activate it, thus leaving you with an expensive paper weight. I would also note that any Comcast box you see for sale on E-Bay or any other auction site is considered stolen material by Comcast. Not only will they refuse to activate the box, they will require that you turn the box in without compensation.
In theory, you should be able to contact Comcast technical support and find out what specifications are required and then purchase a legitimate set top box that meets those requirements, but actually finding a technician capable for delivering that information may be rather difficult.
A digital ready HDTV will act just the same as an old style analog TV, but it no longer requires the digital to analog converter. A new HDTV will not be able to decrypt the encrypted channels of your Digital Economy subscription. You'd still only be able to get your local channels, plus a few more like TBS, TNT, WGN, etc... (whatever channels Comcast in your area sends unencrypted).
I will say this. There is an alternative solution, but it comes with a high front-end cost. It would probably take a few years to recoup it, but I've been VERY happy with the results.
Ceton InfiniTV4
Watch the video and if it interests you, just ask, I can give you the low-down on it. I've been using mine for over a year.
-Wolf sends