monitor resolution not changing

[David]

I'm running Warty and attempting to add extra resolutions. I've used 
dpkg-reconfigure and restarted X (alt-ctl-backspace), but the new 
resolutions (1200x800 and 1152x864) don't happen. They aren't available in 
the SysConfig menu either, although they ARE in the Monitor section of 
XF86config-4 thus:

Modes           "1200x800" "1152x864" "1024x768" "800x600" "640x480"

What have I missed?

Thanks...

David.