You can choose the aspect ratio of the video for advanced receivers (advanced receivers are models 6200 or better). You can also choose between 4:3 (standard) or 16:9 (Cinema) mode. The highest resolution currently possible for broadcast TV is 1080i.
To change the video format settings on your advanced receiver, on your remote, press Menu ► Settings ► Video Settings ► Video Format. Select the format you want to use.