There are two kind of nVidia drivers. The official, proprietary, drivers and the unofficial, open source ones called "nouveau" (AKA "nv"). The latter version is usually shipped by default with most distributions, and are nowhere near usable for any 3d application.
Downloading the drivers
Some distributions make official precompiled drivers available through their packaging system. As not all of them do, those will not be mentioned here.
The latest stable branch is 180.x. You may find them on the official nVidia website. Occasionally, the page is not up-to-date - you will always find the latest driver vesion on the FTP repository (x86-64 / x86). You will need the one called NVIDIA-Linux-[PLATFORM]-[VERSION]-pkg2.run.
Installing the drivers
You will need the following packages:
- gcc-4.3 (or 4.2, depending on the version used to compile your kernel)
Once you downloaded the correct drivers, stop any X server running through sudo /etc/init.d/gdm stop (or any other greeter application you may use). Then, run the installer you downloaded as root through sudo sh NVIDIA-Linux-[PLATFORM]-[VERSION]-pkg2.run.
Follow the instructions given by the installer.
Xorg 1.6, now shipped by default in some distributions, require nVidia drivers version 180.27 or above. The latest beta version is always recommended. nVidia beta drivers are most of the time stable-quality.