What video card, drivers and monitor are you using? Some Xorg drivers (eg the proprietary ATI fglrx drivers, sometimes) don't like certain resolutions even though the hardware is capable of it and other drivers on the same hardware can use them.
Don't forget that the resolution you want to use by default comes first, for example
Viewport 0 0
Modes "1600x1200" "1024x768" "800x600" "640x480"
And don't forget that that XOrg's autodetection can be pretty useless at times, so you may want to specify the monitors HorizSync and VertRefresh manually, like in this example:
Option "VendorName" "CTX"
Option "ModelName" "PV505"
HorizSync 31.5 - 59.0
VertRefresh 60.0 - 70.0
Option "DPMS" "true"
The correct values for your monitor will be in it's manual, which should be available at the manufacturer's website.
EDIT: sometimes you need to use custom modelines to get monitors to display a certain resolution. I am afraid explaining how to use them is beyond my skills at the moment.
EDIT: Could you post your xorg.conf? That might help in traking down problems.