I have Ubuntu 10.04 installed on a Dell C600 and the highest available resolution is 800x600. From my previous I remember 1024x768 would work after setting the colour depth to 16bit, but there is no xorg.conf in /etc/X11 anymore. So, how can the colour depth be changed without a xorg.conf file?
This has been answered at the Ubuntu.SE beta,
You can create a new xorg.conf by switching into a virtual virtual console (Ctrl + Alt + (1-6)) and running sudo service gdm stop.
Then run Xorg -configure (yes, it should be Xorg, not xorg). If you had an old xorg.conf file in /etc/X11/ you'd first back that up by doing sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup.
Then move your newly created xorg.conf to /etc/X11/ by running sudo mv xorg.conf.new /etc/X11/xorg.conf and restart gdm by running sudo service gdm start.
Then you can change the color depth in there by finding the appropriate section and changing/adding whatever's in there to DefaultDepth 16