try letting X reconfigure itself, which should involve steps like the following:

stop X (Ctrl-Alt-Backspace)
# X -configure
# X -config xorg.conf.new
leave the test screen (Ctrl-Alt-F1)
# mv /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
# mv xorg.conf.new /etc/X11/xorg.conf
# xinit
# startx
open your resolution settings and see if new ones appear