Some xorg.conf tips
Virtual xdim ydim
This optional entry specifies the virtual screen resolution to be used. xdim must be a multiple of either 8 or 16 for most drivers, and a multiple of 32 when running in monochrome mode. The given value will be rounded down if this is not the case. Video modes which are too large for the specified virtual size will be rejected. If this entry is not present, the virtual screen resolution will be set to accommodate all the valid video modes given in the Modes entry. Some drivers/hardware combinations do not support virtual screens. Refer to the appropriate driver-specific documentation for details.
Subsection "Display" Depth 8 Virtual 1280 1024 EndSubsection
If your resolution is giving issues under xfce4, try deleting