Step 1: Preparing your package sources

Ensure you have main contrib non-free in your /etc/apt/sources.list since
the nvidia drivers are in non-free and some of the helpers are in contrib.

Step 2: Ensure your kernel is the right version.

The nvidia driver requries a 2.4 or 2.6 kernel at the moment. This should
not be a problem since all current Debian releases use either 2.4 or
2.6 kernels.

Please make sure you are running an up to date kernel so that the headers
matching it can be installed. The simplest way to do this is to do:
> apt-get install kernel-image-2.4-686
or
> apt-get install kernel-image-2.4-k7
or
> apt-get install kernel-image-2.6-686
or
> apt-get install kernel-image-2.6-k7

That will keep you running the latest kernel released (either 2.4.x
or 2.6.x). If the command to install kernel-headers fails later, you
didn't do this step right.

Remember to reboot after installing a new kernel so that you are running
the right one. If you already had the current kernel, you can just
continue.

Step 3: Installing needed packages for building the nvidia kernel module.

Install a few packages needed:
> apt-get install nvidia-kernel-common module-assistant

If you are running a debian kernel do this:
> m-a -i prepare
If you built your own kernel, just make sure you know where the source
for it is located so you can pass the location to m-a.

Step 4: Building and installing the nvidia kernel module

Build the kernel module package:
> m-a a-i -i -t -f nvidia-kernel
> depmod -a
or if using your own kernel build:
> m-a a-i -i -t -f -k /usr/src/kernelsourcepath nvidia-kernel
> depmod -a
Where /usr/src/kernelsourcepath is the location of your kernelsource

Step 5: Installing and configuring the nvidia glx X driver

Install the last few packages:
> apt-get install nvidia-glx nvidia-glx-dev xserver-xfree86

Now you are ready to change X to use the new driver. Configure X
normally using:
> dpkg-reconfigure xserver-xfree86
And configure the monitor, resolution, mouse, etc settings as you need
and select nvidia instead of nv as your driver. If you ever edited the
XF86Config-4 file manually, the simplest way to overwrite it with one that
has the new values is to run 'dexconf'. If you never edited it yourself,
it should just auto update with the new settings.

Note: Any user that wants to run 3D code must be in the video group.
To add a user to the video group do this:
> adduser youruser video
The next time they login to X they will have 3D access.

That's it. You should be running the new X. You should see the nvidia
logo when X starts if you are succesfully running the new driver.

Notes:

Keep in mind that whenever a new version of the nvidia driver comes out,
or whenever you upgrade to a newer kernel, you will have to repeat step
4 again to build the nvidia kernel module to match the new driver and/or
new kernel. One potential solution to part of this (at least for a new
kernel version) would be to have something in the boot up scripts do:
modprobe nvidia || m-a a-i -i -t -f nvidia-kernel && depmod -a && modprobe nvidia
That should load the nvidia driver and if it fails, run the rebuild
command and then load it again. It won't help if a new nvidia glx driver
is installed, since the old module will load fine, but the driver won't
get along with it, and the boot scripts can't detect the cause of X not
started (it probably won't even know X failed to start).

If the nvidia modules doesn't load automatically and the X server
complains about it, add 'nvidia' to /etc/modules to make it auto load
on boot.

When upgrading, the simplest is to reboot afterwards to make sure X and
the kernel module are using the same version. You could also stop X;
modprobe -r nvidia; modprobe nvidia; start X again.