cannot get anything but 640x480

[Raphael]

Hi, I cannot get my monitor to display anything except 640x480 on my 
monitor. I have played with the xconfig file for a few weeks now with no 
luck. Anytime I take the verticle refresh rate off of anything but 100 
nothing is diplayed (sometimes black sometimes a striped pattern of lines).
G4 450mhz
Radeon (one of the first available for the mac) 32 mb
512 mb of ram
17 inch Supermatch XL
I had no trouble setting this machine up for 800x600 under YDL 2.3 (I am 
  contemplating going back).
Any assistance would be greatly appreciated. or if someone has a 
similiar setup could you please email me a copy of your xconfig file, 
that would be great. Thanks,

Raphe