LinuxQuestions.org - Problem changing the video adapter setting and changing the screen resolution

what video card are you using? xf86config edits a file located at /etc/X11/XF86Config or sometimes that ends with a -4 depending on your distro, use autocompletion (press the tab button) to see what it actually is, this is the section relevant to your resolutions themselves, its mostly self explanatory set your default color depth to 24 then use that subsection to put in relevant resolutions and switch between them with cntrl+ or cntrl-

# **********************************************************************
# Screen sections
# **********************************************************************

# Any number of screen sections may be present. Each describes
# the configuration of a single screen. A single specific screen section
# may be specified from the X server command line with the "-screen"
# option.
Section "Screen"
Identifier "Screen 1"
Device "** ATI Radeon (generic) [radeon]"
Monitor "My Monitor"
DefaultDepth 24

Subsection "Display"
Depth 8
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 16
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 24
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
EndSection

as for the correct drivers this will depend on your card but the section looks like this

# **********************************************************************
# Graphics device section
# **********************************************************************

# Any number of graphics device sections may be present

# Standard VGA Device:

Section "Device"
Identifier "Standard VGA"
VendorName "Unknown"
BoardName "Unknown"

# The chipset line is optional in most cases. It can be used to override
# the driver's chipset detection, and should not normally be specified.

# Chipset "generic"

# The Driver line must be present. When using run-time loadable driver
# modules, this line instructs the server to load the specified driver
# module. Even when not using loadable driver modules, this line
# indicates which driver should interpret the information in this section.

Driver "vga"
# The BusID line is used to specify which of possibly multiple devices
# this section is intended for. When this line isn't present, a device
# section can only match up with the primary video device. For PCI
# devices a line like the following could be used. This line should not
# normally be included unless there is more than one video device
# intalled.

# BusID "PCI:0:10:0"

# VideoRam 256

# Clocks 25.2 28.3

EndSection

# Device configured by xf86config:

Section "Device"
Option "EnableDepthMoves" "True
Option "hw_cursor"
#!Option "NoAccel"
Option "AGPMode" "4"
Option "AGPFastWrite" "True"
Option "EnablePageFlip" "True"
Option "UseFBDev" "True"
Option "DPMS" "True"
Identifier "** ATI Radeon (generic) [radeon]"
Driver "radeon"
#VideoRam 32768
# Insert Clocks lines here if appropriate
EndSection

please post again if you need any clarification or further help