I have two comcast HD motorola boxes as well as a cable card. It seems I have a weak signal coming in seeing how it is difficult for all three (2 HD Boxes & Cable Card) to get great HD reception.

I am wondering if it is true if Comcast can increase/decrease "My" signal before it enters my house (i.e. @ their box)

Call them up, they can send someone out with testing equipment. They should be able to confirm signal strength issues, and install a Distribution Amplifier somewheres around where the line comes into the house.

Distribution Amplifier = gizmo that sits in-line with the cable and uses electricity to boost the signal. There should be no charge for this.