Comcast HD box has no HDMI slot. Can I get 1080 with component cables? Or do I have to get another box?

I have a 42 inch sanyo hdtv but see no increase in clarity. I bough HDMI cables, but then realized that there is no slot in my comcast box.I have read that if I turn off the cable box, then hit menu I can go in and switch my signal to 1080, and that component cables will carry the signal. As of now I see no difference between my HD channels and regular channels. Do I need to go get another box with an hdmi slot to carry the signal to see a difference? Will I be charged more each month for rental?