xrandr is responsible for this.
I also run arch64 with the same setup as you and the same resolution, quite interesting. :p
Anyways, what you need to do is to disable twinview. Type xrandr to show the currently available resolutions.
Code:
Identifier "Screen0"
Device "Device0"
Monitor "Monitor0"
DefaultDepth 24
[b] Option "TwinView" "0"[/b]
SubSection "Display"
Depth 24
Modes "1680x1050_60.00"
EndSubSection
EndSection
Code:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce 8800 GT"
Option "NoLogo" "True"
Option "RenderAccel" "True"
Option "NoRenderExtension" "False"
Option "NvAGP" "0"
Option "AddARGBGLXVisuals" "True"
Option "AllowGLXWithComposite" "True"
[b] Option "DynamicTwinView" "False"[/b]
EndSection
The twinview parts on both parts are the lines you need. This worked for me, I now have loads of resolutions to pick in game as opposed to when twin view was enabled.
