"What you want to do is change the GDM LOGIN Screen resolution?
Read this and please accept the correct answer.
Justin was kind of correct except the fact that GDM uses the modeline from your xorg.conf. The priority runs from left to right, so if you add ""1280x960"" as the first entry and remove any items you don't want (who needs half of them anyway, 768x600 or whatever it was, madness.) you should be fine for GDM.
"If you don't have a special X11 configuration but you do happen to have a file /etc/X11/xorg.conf, you might want to try moving it aside and see what happens. Usually X11 should configure itself properly these days and in my experience having stuff in /etc/X11/xorg.conf often causes problems.
