Quando non viene riconosciuto il monitor, Ubuntu (ma penso qualsiasi distribuzione) imposta la risoluzione dello schermo a 800 x 600.

per modificarla, dobbiamo editare il file Xorg.conf e aggiungere:

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 "Emulate3Buttons" "true"
EndSection

Section "Device"
Identifier "ATI Technologies Inc RV370 5B60 [Radeon X300 (PCIE)]"
Driver "vesa"
# BusID "PCI:1:0:0"
EndSection

Section "Monitor"
Identifier "Monitor Generico"
Option "DPMS"
HorizSync 28-51
VertRefresh 43-60
EndSection

Section "Screen"
Identifier "Default Screen"
# Device "ATI Technologies Inc RV370 5B60 [Radeon X300 (PCIE)]"
Monitor "Monitor Generico"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1152x864" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection

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

il sistemo l'ho testato su ubuntu 8.04 con EMC"  installato su macchina virtuale Virtualbox

ad ogni modo un xorg.conf.completo è il seguente XORG.CONF