I have just installed Xubuntu for the first time.

My Dell FPW2405 LCD monitor is not being detected properly. Its 1920x1200 native resolution is not available. The highest I can go is 1600x1200. I know exactly what to add to an existing xorg.conf in order to get it to work (a modeline in the "Monitor" section and a reference to this mode in the "Screen" section), but my installation of Xubuntu (9.10, 64-bit the latest) does not come with an xorg.conf file. I am currently using the open-source ati driver and I would like to keep it that way.

How do I generate an xorg.conf file? Especially one that will work! Or is there another way to do this?