you can do this, but there you wont get the best signals. Your best best is hook up anything HDMI you can. But if your projector only has component that is as far as you can go. I would look at the specs on your projector. If you can display 720p that's awesome.
1. take comcast HD box remote, turn box off, press menu button, this gets you to a hidden menu
2. find where it says 1080i and make it 720p. this will give you max resoltion that box can do. you can use component cables with this
With the Dvd, you are pretty much out of luck, you are going to have to hook it up component and it will not upconvert to 1080i hd video. If you can look out there somewhere and find a scaler or something with mulitple HDMI inputs and make sure that it wont lose any decent signals, get that.
