[CentOS] x display issue

Hello,
    I've got a new Centos5 install and got x installed with gnome on it. The 
problem is the display looks awful and keeps flickering. It failed the 
monitor probe and doesn't have horizontal and vertical refresh rates in the 
xorg.conf file. I believe this is part or all of the issue, i took it down 
to 8-bitt color and 800x600 resolution, that didn't help. The video card is 
an sis 741/760 AGP integrated job, the monitor has no designation, except 
compaq. Does anyone know how i can find the values i need?

Thanks.
Dave.