I had the same problem with my nvidia card setup - you have to make sure in xorg.conf that you have your monitor settings in there so you can change the resolution higher than 1280x1024. I.e. /etc/X11/xorg.conf:
VendorName "Monitor Vendor"
ModelName "Monitor Model"
HorizSync 28.0 - 78.0
VertRefresh 50.0 - 76.0
Make sure to set the Horizontal Sync and Vertical Refresh to the correct settings for your monitor otherwise you can cause damage but if they are there that should let you select more resolution options.
And if it still doesn't let you change it I just did this cause I'm too lazy to use configuration windows.
Viewport 0 0
in your case whatever size resolution you want -> 1920x1080?
I don't have any modeline settings in my xorg.conf it somehow does it automagically when you add HorizSync and VertRefresh.