Oui, il faut modifier le "xorg.conf".
Pour le driver
Section "Device"
Identifier "Videocard0"
Driver "vesa"
...
Pour la résolution
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 16
SubSection "Display"
Viewport 0 0
Depth 16
Modes "800x600" "640x480"
EndSubSection
EndSection