I have comcast HD also. The highest resolution for broadcast tv is 1080i. You may have to go into a menu to set this up. To reach this menu, turn the box OFF and have your tv on. From there, hit the "menu" button on the front of the box. This will bring up your user settings. Here, you can set your resolution to 480i/p, 720p, or 1080i.
It wont look as good as a blue ray because of the bandwith, but it should look pretty damn good. My best channels are Discovery, ESPN, NatGeo, and HGTV believe it or not. They look like they are using high def cameras to shoot shows instead of just upscaling like some other channels do. NBC's Heros looks nice in High Def as well as some Fox shows (Terminator, House, etc...) Try that and see if you can get a better pic.