CentOS 5 Hardware :: No Graphics On Dell Optiplex 780?

I have a brand new OptiPlex 780 from Dell, I downloaded CentoOS x86_64 from [URL]. The installer runs the graphics mode OK, when I reboot into normal, the graphics doesn't work.

The last messages I see are:

Red Hat nash version 5.1.19.6 starting
starting clock,
starting UDev

then the display goes away, the montior says "Not Support" The monitor is 1 day old and supports VGA to 1680x1050.

I try pressing "I" (and/or "i") when Red Hat first starts, but the interactive menu doesn't show.

I have two idenical machines, the other is running XP (the CentOS was also before I loaded CentOS), WinXP is allowing screen resolutions of

- 800 x 600 @ 60Hz, 72Hz, or 75Hz
- 1024 x 768 @ 60Hz, 70Hz, or 75Hz
- 1280 x 1024 @ 60Hz or 75Hz
- 1680 x 1050 @ 60Hz
...all working on the same monitor.

During Bios boot, the monitor shows 600x800 @ 60Hz, during the text-only start of CentOS, it shows 720x400 70Hz. After CentOS boot shows starting udev, the montior breifly shows 720x400 70Hz, then "Not Support".

So....
Why isn't CentOS using the same graphics mode it used during installation (that worked fine)