I recommend you get the Digital Video Essentials DVD and use one of the geometry/overscan patterns you can find on there. You will also need to search the web to find out how to get into the factory menu of your TV.
Make a note of the defaults first, in case you screw the adjustments up!
You should push the picture as far as it will go without distorting the geometry at the edges. In most cases you should be able to limit overscan to less than 2.5%.