Only use part of widescreen monitor

[Noven]

I'm wondering if therer is some way to convince Xorg to not use the entire monitor. I have a laptop with a 1280x800 screen. Works perfectly under most circumstances. However some fullscreen games can't use an appropriate resolution - they end up either stretched or (in the case of 640x480 games) only using a box in the center or top left of the screen, leaving the rest black. I'd like to set up Xorg parameters to tell it to launch a screen (say 1024x768) with black bars at the side where screen is not used rather than stretching to fill the available monitor. Is that at all possible?
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