Some programs in Linux (including the ultra-cool Desktop Effects) need graphics accelleration, so they need to use your card to the full. You need to install your graphics drivers if you want to play 3d games.
First open the terminal (Applications, Accessories, Terminal)
If you have a recent card type this and hit enter:
sudo apt-get install nvidia-glx-new
sudo apt-get install nvidia-glx
If you have an older card (only for TNT, TNT2, TNT Ultra, [old] GeForce, and GeForce2 chipsets):
sudo apt-get install nvidia-glx-legacy
For ATI, enter this in the terminal and hit enter:
sudo apt-get install xorg-driver-fglrx
After your graphics card is all set up you’re set to have fun.
Play around with desktop effects by installing this package:
sudo aptitude install compizconfig-settings-manager
Then go to the System menu, click on Preferences, and then click on Advanced Desktop Effects Settings, and have fun!