If all else fails, try to start X "bare"; that is, use a command, such as:

X > /tmp/x.out 2>&1

You can then kill the X server (using the Ctrl-Alt-Backspace key combination) and examine the contents of /tmp/x.out. The X server reports any warnings or errors — for example, if your video card doesn't have a dot clock corresponding to a mode supported by your monitor. This output can be very helpful in diagnosing all kinds of problems. Examine it closely if your X server does not start up at all, does not provide the resolutions you wanted, or shows a flaky, snowy, or otherwise insufficient picture. Even if everything works to your satisfaction, you might want to check this file for interesting information that the X server has found out about your hardware. The lines starting with (**) contain data that you provided yourself in the configuration file, while lines starting with (- -) contain data that the X server has found out itself.

Remember that you can use Ctrl-Alt with the plus or minus keys on the numeric keypad to switch between the video modes listed on the Modes line of the Screen section of XF86Config. If the highest-resolution mode doesn't look right, try switching to lower resolutions. This lets you know, at least, that the configurations for those lower resolutions in your X configuration are working correctly.

Also, check the vertical and horizontal size/hold knobs on your monitor. In many cases it is necessary to adjust these when starting up X. For example, if the display seems to be shifted slightly to one side, you can usually correct this using the monitor controls.