That override setting you found tells the dvr what to do with standard format (480i and p) television. I am sure that you have noticed now that you have to use your televisions zoom/stretch/wide screen feature to get a standard definition program to completely fill the screen.
I trouble shoot Motorola dvr's all day, every day; I even get to install them on occasion. On every one that we set up for HD, we set them to 1080I and the override to 480i or p, whichever the TV will do. If we don't, our cust's get ticked that the picture on a SD program does not fill their TV screen.