Display Problems

[John]

Monitor = Viewsonic G790
Video card = Ge Force 8400 GS
System = Fedora Development (Rawhide) updated through 7/29/09.


Up to Fedora 11, the monitor could be run at a resolution of 1600x1200; 
with ".fc12" the maximum resolution is 1152x864.

Any changes I make in system-config-display are not recognized. Find 
nothing of note in the error logs.

What do I have to do to use the monitor at 1600x1200?

TIA.

John