[ALUG] Why does Ubuntu set my resolution to 1152x864?

I'm trying out Ubuntu 8.10 on my new system. It insists on setting the resolution in X to 1152x864 by default. This is a real pain because it looks awful on my Dell display which has a native resolution of 1600x1200. I can set the user desktop to 1600x1200 but that doesn't change anything else, for example the gdm login is still at the lower (fuzzy) resolution and if I change to anothe window manager that's at the lower resolution too. Where does the 'default' come from now that there are no explicit resolutions in xorg.conf? Can I put 1600x1200 explicitly in xorg.conf and, if I do that, will it cause any problems? -- Chris Green