Option "NoLogo" "False"
Option "RenderAccel" "On"
Option "HWcursor" "On"
Option "DamageEvents" "True"
Option "TwinView" "0"
Option "metamodes" "DFP-0: 1680x1050 +0+0"
First, it would be good if you would add the 'metamodes' line as I have in the above 'Screen' section. In your case, with only one monitor, changing 'metamodes' to 'modes' will probably do.
Disregard the other stuff in there, I just copied & pasted
PS - Dunno what made me think that was a laptop
but thanks for the clarification.
Note too, checking your xorg.0.log file can lead to clues as to what's wrong, but for now, try it with a MODES line as I indicated, and see if it changes anything.
For reference, always remember the driver docs that come with your driver, and the man page for the driver too, for exact syntax of stuff.
Good luck! I'll check later and see what's up with your situation.