select your GPU / Monitor and set the GPU Scaling Method to "Centered" or "Aspect Ratio Scaled".
if those changes disappear after restarting your X server (i.e. rebooting), then make sure
is run after starting X.
edit: hmm.. wait. I think that applies to DVI connections only. With VGA connectors, you'll probably need to change the setting on your monitor, since VGA connectors don't transmit scaling information. If the monitor doesn't support that, bad luck.