Again, I don't understand why you would bother with Comcast unless the Boston Channels are broadcasting in HD. If you get the local channels package from DirecTV and have the DirecTV HD package (which it sounds like you do), then you get the local channels IN HD as long as the local channels broadcast in HD.
In other words, I think you can save yourself about $40 a month by dumping Comcast and just going with the local channels package from DirecTV. Unless there is another reason why you have Comcast.