First check to see if you have the higher rez available, go to System > Preferences > Screen Resolution
see if you can change it from there. If the rez you want is not listed then you will need to edit your /etc/X11/xorg.conf file and add the one you want.
Modes "1280x1024" "1024x768" "800x600"
Add the Modes you want, the default mode will be the first one, the file is read from left to right. So in this one shown "1280x1024" will be default at boot time, unless you change that in Gnomes rez program first mentioned.