Change to screen resolution 1280x800 on Slackware 12

-Calculate VESA GTF mode lines with gtf command
#gtf 1280 800 60

# 1280x800 @ 60.00 Hz (GTF) hsync: 49.68 kHz; pclk: 83.46 MHz
Modeline "1280x800_60.00" 83.46 1280 1344 1480 1680 800 801 804 828 -HSync +Vsync

-Copy the result to section Monitor in /etc/X11/xorg.conf


Section "Monitor"

Identifier "My Monitor"

HorizSync 28-64

VertRefresh 40-90

Modeline "1280x800_60.00" 83.46 1280 1344 1480 1680 800 801 804 828 -HSync +Vsync
EndSection

- Also add the following in the Screen section
Section "Screen"
Identifier "Screen 1"
Device "VESA Framebuffer"
Monitor "My Monitor"

# 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 "1280x800" "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 16
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 24
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 32
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubsection

EndSection

-Check the result with xrandr
$ xrandr
Screen 0: minimum 640 x 480, current 1280 x 800, maximum 1280 x 800
default connected 1280x800+0+0 0mm x 0mm
1280x800 75.0*
1024x768 76.0
800x600 86.0
640x480 86.0

you might require to reboot