Definitely check out whatever program came with your graphics card that will allow you to tweak its performance. Don't know what hardware you have, but on my rig even when I set everything up with the mod to run at 1400x1050, running the game fullscreen still resulted in it being stretched out to fill the monitor. So I had to go in and tell the gfx card not to do that and to maintain the aspect ratio that I'd set up with the res fix. Since then its worked like a charm.
Good luck!