NVIDIA on Ubuntu :

NVIDIA drivers work great on Ubuntu, as long as you know how to install them. If you have have set up your resolutions correctly its easy as pie.
First check if you need the NVIDIA-Legacy driver or not.
If you are sure about the driver and have the universe and multiverse repositories set up its time to install the NVIDIA driver.
First make a back up of your xorg.conf :

sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.bak

Now install the driver:

sudo apt-get install nvidia-glx

If you have an older card, install the legacy driver instead:

sudo apt-get install nvidia-glx-legacy

Now all you need to do is: on Egdy-Eft (6.10)

sudo nvidia-xconfig

On older versions of Ubuntu; Dapper Drake and before:

sudo nvidia-glx-config enable

now do

sudo /etc/init.d/gdm stop
sudo /etc/init.d/gdm start

(on Kubuntu its kdm , not gdm)
Now your nvidia is enabled, with:

glxinfo | grep direct

You should see:

direct rendering: Yes

If you run into problems:

sudo cp /etc/X11/xorg.conf.bak /etc/X11/xorg.conf

all should return to the old situation and no harm done .
The latest Nvidia driver can also be installed with Envy, read this post!