X11 config for Dell Inspiron on Ubuntu and Linux Mint where maximum screen resolution is only 800x600 you can add 1024x768 by using this configuration file.

xorg.conf

# Install:
#
# On Mint 6 Fluxbox edition this file goes in /etc/X11/xorg.conf
# On Mint 10 LXDE edition this file goes in /usr/share/X11/xorg.conf.d/20-screen.conf
# This config will also work for versions of Ubuntu of course in /etc/X11/xorg.conf
#
# Don't forget to back up your original xorg.conf file before adding this one.
#
# You'll need to restart your machine to see the effect.



# xorg.conf (X.Org X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# Note that some configuration settings that could be done previously
# in this file, now are automatically configured by the server and settings
# here are ignored.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
#   sudo dpkg-reconfigure -phigh xserver-xorg

Section "Device"
	Identifier	"Configured Video Device"
EndSection

Section "Monitor"
	Identifier	"Configured Monitor"
	VendorName	"Monitor vendor"
	ModelName	"Monitor model"
	HorizSync	30-81
	VertRefresh	58-75
EndSection

Section "Screen"
	Identifier	"Default Screen"
	Monitor		"Configured Monitor"
	Device		"Configured Video Device"
	DefaultColorDepth	16
SubSection "Display"
	Viewport	0	0
	Depth		1
	Modes "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
	Viewport	0	0
	Depth		4
	Modes "1024x768" "800x600" "640x480"
EndSubsection
SubSection "Display"
  Viewport 0 0
  Depth 8
  Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
  Viewport 0 0
  Depth 15
  Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
  Viewport 0 0
  Depth 16
  Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
  Viewport 0 0
  Depth 24
  Modes "1024x768" "800x600" "640x480"
EndSubsection
EndSection