You would need to buy any AGP 2x compatible card for your model's slots which are, I believe, PCI. There are several ATI cards that should work well such as the ATI 9600 or 9800, for example.
Installation is simply the removal of the existing card and replacing it with a new one. There will also likely be some driver installation needed, too. You'll need a small Phillips screwdriver to remove the screw that holds the card in the slot. Of course you will need to remove the side panel to access the interior.