1280x1024 has to be at the front of the list in order to work it should look something like this:
Code:
Section "Screen"
Identifier "Screen 1"
Device "nVidia GeForce 7800 GTX"
Monitor "Samsung SyncMaster 710T"
DefaultDepth 24
Subsection "Display"
Depth 8
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 16
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 24
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
EndSection