Well, it depends on whether you use stretched monitor resolutions. If your CRT has a 4:3 aspect ratio, you should only use 4:3 resolutions like 800x600 or 1024x768. If your TFT is 5:4 (like my Hyundai ImageQuest Q17), you should not select 4:3 resolutions like 1280x960, but use the 5:4 one: 1280x1024.
If you ahve a TFT and want to select full screen resolutions with a different aspect ratio, you should deactivate the stretch to full surface option in your video driver sothat the image is centered with black borders around it, but with the correct aspect ratio. There are also video drivers that allow stretching the dislayed image with an aspect ratio, but the image won't look as sharp as when using no stretching at all in the drivers.