xorg.conf

なんとか 1680x1050 出力できました
xorg.conf の変更点は以下です

赤部分を追記
Section "Device"
Identifier "Videocard0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce FX 5200"
Option "AddARGBGLXVisuals" "True"
Option "NoLogo" "True"
Option "ModeValidation" "DFP: NoDFPNativeResolutionCheck, NoVertRefreshCheck, NoMaxPClkCheck, NoHorizSyncCheck, NoEdidMaxPClkCheck, AllowInterlacedModes, NoMaxSizeCheck"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
Option "TwinView" "0"
SubSection "Display"
Depth 24
Modes "1680x1050"
EndSubSection
EndSection