a) you should not start X with sudo X. Use gdm or run the startx script as a normal user instead.
b) please provide us some information about your system
- graphics card (lspci)
my max resolution is 1240x1040 (or 1280x1080) and I always use that
I have never seen a monitor with those resolutions. If you are sure these resolutions are correct, you will need to add a modeline to your xorg.conf.