questo è il mio xorg.conf, ma come si vede le risoluzioni sono messe bene.
codice:
Section "Files"
EndSection

Section "InputDevice"
	Identifier	"Generic Keyboard"
	Driver		"kbd"
	Option		"CoreKeyboard"
	Option		"XkbRules"	"xorg"
	Option		"XkbModel"	"pc105"
	Option		"XkbLayout"	"it"
EndSection

Section "InputDevice"
	Identifier	"Configured Mouse"
	Driver		"mouse"
	Option		"CorePointer"
	Option		"Device"		"/dev/input/mice"
	Option		"Protocol"		"ImPS/2"
	Option		"ZAxisMapping"		"4 5"
	Option		"Emulate3Buttons"	"true"
EndSection

Section "InputDevice"
	Driver		"wacom"
	Identifier	"stylus"
	Option		"Device"	"/dev/input/wacom"
	Option		"Type"		"stylus"
	Option		"ForceDevice"	"ISDV4"		# Tablet PC ONLY
EndSection

Section "InputDevice"
	Driver		"wacom"
	Identifier	"eraser"
	Option		"Device"	"/dev/input/wacom"
	Option		"Type"		"eraser"
	Option		"ForceDevice"	"ISDV4"		# Tablet PC ONLY
EndSection

Section "InputDevice"
	Driver		"wacom"
	Identifier	"cursor"
	Option		"Device"	"/dev/input/wacom"
	Option		"Type"		"cursor"
	Option		"ForceDevice"	"ISDV4"		# Tablet PC ONLY
EndSection

Section "Device"
	Identifier	"nVidia Corporation NV5M64 [RIVA TNT2 Model 64/Model 64 Pro]"
	Driver		"nv"
	BusID		"PCI:1:0:0"
EndSection

Section "Monitor"
	Identifier	"AL1716"
	Option		"DPMS"
	HorizSync	30-83
	VertRefresh	56-75
EndSection

Section "Screen"
	Identifier	"Default Screen"
	Device		"nVidia Corporation NV5M64 [RIVA TNT2 Model 64/Model 64 Pro]"
	Monitor		"AL1716"
	DefaultDepth	24
	SubSection "Display"
		Modes		"1280x1024" "1280x768" "1024x768" "800x600" "640x480"
	EndSubSection
EndSection

Section "ServerLayout"
	Identifier	"Default Layout"
	Screen		"Default Screen"
	InputDevice	"Generic Keyboard"
	InputDevice	"Configured Mouse"

# Uncomment if you have a wacom tablet
#	InputDevice     "stylus"	"SendCoreEvents"
#	InputDevice     "cursor"	"SendCoreEvents"
#	InputDevice     "eraser"	"SendCoreEvents"
EndSection