I think this is overscan, something desirable for video or multimedia
output but not for desktop uses (the same rule applies to the svideo
conection and pal or ntsc TVs, they are good for video and multimedia
but not for desktop uses).
In case anyone else is using svideo for the desktop:
Since asking about this I got it to work on LXDE desktop environment,
as it has a setting for margins (in the openbox menu option).
Changing the X configurations does not work (X won't let me lower the
resolution if it can't detect the monitor itself).