1280x1024 on Ubuntu with Nvidia Geforce 8400 GS

The solution with an undetected monitor is usually this:
1- Find somewhere the actual supported refresh rates for your monitor
(for example you can find from that page: http://www0.shopping.com/xPF-BenQ-BenQ-19-LCD-T905-TCO99-Monitor-Silver-Black-12ms that the vertical range is 56-76 and the horizontal range is 31-81).
2- Edit the /etc/X11/xorg.conf file in section "Monitor" to use those ranges:
Section "Monitor"
Identifier "Monitor0"
VendorName "Unknown"
ModelName "CRT-0"
HorizSync 31.0 - 81.0
VertRefresh 56.0 - 76.0
EndSection
3- Restart X or your computer
4- Run nvidia-settings and the resolution you're looking for should be there.

http://superuser.com/questions/60290/cant-set-1280x1024-on-ubuntu-with-nvidia-geforce-8400-gs