The best way to use headless with your current video driver (instead of vesa) is to use "CRT-0" monitor:
Section "Screen"
Identifier "Screen0"
Device "Device0"
Monitor "Monitor0"
DefaultDepth 24
Option "ConnectedMonitor" "CRT-0"
Option "UseEDID" "FALSE"
SubSection "Display"
Depth 24
Modes "1280x1024"
EndSubSection
EndSection
The driver will try to determine valid modes and while it is the CRT-0, the driver will fail to obtain mode and will use the default (which is 800x600)
So you need to explictly tell the driver to use "1280x1024" which is defined in the Monitor0 section as:
Section "Monitor"
Identifier "Monitor0"
VendorName "Fake Vendor"
ModelName "Fake CRT"
HorizSync 28.0 - 80.0
VertRefresh 48.0 - 75.0
ModeLine "1280x1024" 108.88 1280 1360 1496 1712 1024 1025 1028 1060 -h sync +vsync
Option "DPMS"
EndSection
Restart X and voila!