You can set your resolution in Modes line: Section "Screen" -> SubSection "Display" -> Modes
For.Ex.:
Code:
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1280x1024"
EndSubSection
EndSection
There could be several SubSections "Display" for differnet depths, DefaultDepth line is used to switch them.