First,install Envyng which is a simple GUI tool that can be used to install/uninstall Video driver for your ubuntu.
Just use one command in terminal(Applications/Accessories/Terminal):
sudo apt-get install envyng-core envyng-qt
This will properly installed envyng with both GUI mode and text mode.
Go to Applications->SystemTools->EnvyNG.
Alternatively,type this command in terminal:
and type a number in this screen:
Update:By now,envy isn’t available in Ubuntu 10.04(Lucid).You can read this howto about install nvidia driver in ubuntu 10.04