Maybe not an answer, but some questions:
Why don't you just use vesa?
Is the Xorg server starting, but the screen is staying black? Or do you get an error in /var/log/Xorg.0.log? (You can find the previous log in /var/log/Xorg.0.log.old.)
What resolution are you aiming at?
When you say VGA, you really mean 640x480, 60 Hz? In that case you should set 'HorizSync 31-32' (and perhaps 'VertRefresh 59-61') in the "Monitor" Section.
But if your monitor in multisync (as they have been for 20 years), the problem is probably in the "ati" Driver.
The Driver "ati" should work with your graphics chip.