Comment 26 for bug 162136

(In reply to comment #11)
> I search for randr with gconf-editor but found only two entries which I think
> are unrelated. I think this setting is creating problems at my end. Would you
> mind helping me to find that key?

~/.gconf/desktop/gnome/screen/*

>
> I am still completely stuck with the newer Xorg on Thinkpad X24. Latest output
> from "xrandr -q" is now again back to
>
> $ xrandr -q
>
> Screen 0: minimum 320 x 200, current 1024 x 768, maximum 1280 x 1200
> VGA-0 connected (normal left inverted right)
> 1280x1024 74.9 75.0 59.9
> 1152x864 75.0 74.8
> 1024x768 74.9 75.1 70.1 60.0
> 832x624 74.6
> 800x600 72.2 75.0 74.9 60.3
> 640x480 75.0 72.8 66.7 60.0
> 720x400 70.1
> 640x350 70.1
> LVDS connected 1024x768+0+0 (normal left inverted right) 0mm x 0mm
> 1024x768 60.0*+ 60.0
> 800x600 60.3
> 640x480 59.9
>
> (resolution of external monitor is not correctly discovered.)
>

it is, but as I said, the max size of your desktop is not big enough to accommodate it:
(II) RADEON(0): Not using mode "1680x1050" (width too large for virtual size)

you'll need to add a virtual line (like your previous config) to the display subsection of the screen section your config to allocate a large enough desktop:
Virtual 2704 1050