I had the same problem in Feisty and to get the full resolution had to
edit the following lines of /etc/X11/xorg.conf file monitor setting;-


Section "Monitor"
Identifier "SyncMaster"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 75.0
Option "DPMS"
EndSection

Section "Device"
Identifier "nVidia Corporation NV18 [GeForce4 MX 4000]"
Driver "nvidia"
EndSection

Section "Screen"
Identifier "Default Screen"
Device "nVidia Corporation NV18 [GeForce4 MX 4000]"
Monitor "SyncMaster"
DefaultDepth 24
Option "AddARGBGLXVisuals" "True"
SubSection "Display"
Depth 1
Modes "1680x1050" "1280x1024" "1280x960" "1152x864"
"1024x768" "800x600" "640x480"