If you've already entered the correct specs for your monitor, you don't want to change those.
Have you tried running Xconfigurator to select higher resolutions? You can select multiple resolutions, and once in X, you can toggle through them on-the-fly by pressing CTRL + ALT + the numeric keypad's "+" or "-" keys.
Also, you can look in your XF86Config-4 file under the "screens" section to see which video server X is using and what resolutions are currently available. Make sure you're using the right server for your particular card, and that there are more resolutions listed than just "640x480".