I take it you have read trough ATI help contents, are you using the latest firmware for your TV.
ati ccc-Manually Adjusting Image Size and Position using Underscan/Overscan for TVs -
1.Based on the navigation system contained in the ATI Catalyst™ Control Center window, do one of the following:
•Navigation menu—From the display palette on the Desktops & Displays page, click the desired TV, and then click Configure in the context menu that appears.
•Navigation tree—From the tree in the Graphics Settings tab, expand TV Properties.
2.Click Adjustments.
3.Move the Underscan/Overscan slider to adjust the overall size of the image on the TV.
Use this slider to adjust any visible black borders around the edge of the image.
Note: The slider is not available for TVs configured to use a custom or the default optimized timing mode.