Does the ATI have it's own software to play with?
In my experience, changing the resolution in media centre will not help, but you could try it in any event - configure the tv or monitor option in tv setup.
Yes I have fiddled with the resolution through the ATI software, lots to fiddle with but I managed in the end to get a good picture by setting both the Media Centre and the graphics card to 1152x864.
This isn't perfect because the TV will now not allow me to set the 'Just Scan' picture size setting. Result is I have to adjust the zoom for each channel depending on how it's broadcast.
Thanks for the help anyway.