I have tried all of the install methods at one time or another. Kudos to the Debian NVidia team for making each time easier than the last. I have less total experience with the ATI cards, also kudos to that team.
If you are running unstable or testing, the *-dkms packages (nvidia-kernel-dkms, nvidia-kernel-legacy-173xx-dkms, or fglrx-modules-dkms) make life really easy – even as you change kernel versions. That is the first recommended solution – if appropriate to your install and hardware – note the drivers only currently support the newest generations of hardware.
Today at work I just used the packages (nvidia-kernel-2.6-686, nvidia-kernel-legacy-96xx-2.6-686) + nvidia-glx + mesa-utils to configure 4 workstations in a few minutes and following the instuctions at http://wiki.debian.org/NvidiaGraphicsDrivers for “Configure X to use the nvidia driver” . The only trick is figuring out which version you want for your card. These 4 were all built with Debian Lenny – aka 5.0.
I have previously used the instructions for module-assistant with success. There were more details there than a beginner would be comfortable with, but not near as bad as doing it yourself.
I did try the steps including downloading the driver from NVidia, but was unsuccessful at that. It is worth trying all of the other solutions first before trying to puzzle through building it yourself. Among other things the other solutions are less likely to break with patching or upgrades (or at least break as messily).