Below is the HD Guru HDTV Seating Distance Chart. It is based on human preception of detail. . Sitting at the optimum 720p distance using a 1080p display will yield no improvement in picture quality due to limits of our vision. In other words, if you plan to sit too far back from a given size 1080p, consider changing to a 720p HDTV or purchasing a larger 1080p HD set. Otherwise you will be wasting money by paying for higher resolution you can’t see.All screen sizes are in diagonal inches and all viewing distances are in inches. An example the 42″ screen size:
To see all the high definition of a 42″ 720p display requires a maximum viewing distance of 98.83 inches or 8.235 feet. If the 42″ display is either 1080i or 1080p the maximum viewing distance for full visual resolution is 65.89 inches or 5.49 feet. If you view at a greater distance you will not achieve full visual resolution of all the detail.
Bottom line, you need to sit closer to a 1080 display than a you would with a 720p HDTV .