Find this section: My screen is 800x600; but if I want to change I can put 1200x768 in front of 800x600 and restart X and that is it. To change it back I place 800x600 in front

Section "Screen"
	Identifier	"Default Screen"
	Device		"ATI Technologies Inc Rage 128 RF/SG AGP"
	Monitor		"Generic Monitor"
	DefaultDepth	24
	SubSection "Display"
		Depth		24
		Modes		"800x600" "720x540" "720x400" "640x480"
	EndSubSection
EndSection