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HOWTO: install an Nvidia card instead of your Intel integrated card
REQUIREMENTS
1) an installed Ubuntu system (don't try it on a livecd)
2) an Nvidia graphic card
3) an Internet connection (better if broadband)
INTRODUCTION
This guide is aimed at the owners of motherboards which have an Intel integrated graphic card and which sometimes can be a bit unfriendly when you try to use a different graphic card in GNU/Linux.
In this guide you will learn how to get disable your onboard card both in the BIOS and in Ubuntu and use your new Nvidia card (it might work with ATI cards as well though)
INSTALLATION
1) Turn on your computer, enter the BIOS and set your graphic adapter BIOS to "onboard".
IF AND ONLY IF that is not possible please DO NOT plug in your Nvidia card (otherwise you might get a black screen).
OTHERWISE you can plug in your Nvidia card just to save time)
2) Make sure that the Monitor cable is connected to the integrated card.
3) Boot Ubuntu and type this in Terminal (or Konsole):
sudo nano -w /etc/modprobe.d/blacklist
Add the following lines to the end of the file:
blacklist agpgart
blacklist intel_agp
Press CTRL+X to exit (save the file)
4) Type:
sudo nano -w /etc/X11/xorg.conf
get to the Section Device of the file (scroll it with your keyboard arrows) and put a "#" before the BUSID of your card like in the example below:
Section "Device"
Identifier "NVIDIA Corporation NV34 [GeForce FX 5500]"
Driver "nv"
#BusID "PCI:1:9:0"
Then add "Option "NvAGP" "1"" below the BUSIID (NOTE: this is not required by ATI users):
Section "Device"
Identifier "NVIDIA Corporation NV34 [GeForce FX 5500]"
Driver "nv"
#BusID "PCI:1:9:0"
Option "NvAGP" "1"
Press CTRL+X to exit (save the file)
NOTE: of course the card displayed in the example might not match yours. There might be your Intel integrated card in there as well.
5) Install the Nvidia Driver
NOTE: if you have an ATI card you can skip this step and set the driver to "ati" in the Section Device of your /etc/X11/xorg.conf
6) Turn off your computer
7) IF AND ONLY IF you did not plug in your card in step 1 follow this step (OTHERWISE just go to step 8 )
Plug in your Nvidia card but DO NOT connect your monitor to it.

Enter the BIOS and set your graphic adapter to 'auto' (or "PCI" or "PCI-E", etc. accoring to what your BIOS enables you to select) in order to use your Nvidia card.
Save the settings and turn off your computer.
9) Connect the monitor to your Nvidia card.
10) Turn on your computer and boot Ubuntu as usual