Hi!
As root, open the file /etc/X11/XF86Config(or XF86Config-4 or Xorg, depending of your distro) for editing. Find some lines similar to this at the "Screen Sections":
Code:
# If your card can handle it, a higher default color depth (like 24 or 32)
# is highly recommended.
# DefaultDepth 8
# DefaultDepth 16
DefaultDepth 24
# DefaultDepth 32
# "1024x768" is also a conservative usable default resolution. If you
# have a better monitor, feel free to try resolutions such as
# "1152x864", "1280x1024", "1600x1200", and "1800x1400" (or whatever your
# card/monitor can produce)
Subsection "Display"
Depth 8
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 16
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 32
Modes "1024x768" "800x600" "640x480"
EndSubsection
...
On the line:
Code:
Modes "1024x768" "800x600" "640x480"
You might want to add 1024x768 as your first choice of resolution. If something goes wrong, 800x600 will be used instead....
Good luck!