Note that XFree86 4.0.1 and later is required with driver 0.9-6 and later. If you have XFree86 4.0.0, then you'll have to download the older 0.9-4 version. You should try to use the lastest XFree86 and NVIDIA drivers.
Download the drivers at: http://www.nvidia.com/
See the updated FAQ at Nvidia.com while you are downloading. It has important installation information not in this HOWTO.
The NVIDIA drivers provide a kernel driver: /lib/modules/kernel.version.number/video/NVdriver and libGL.so and libGLcore.so files that go into /usr/lib/ to replace and Mesa ones that might be in there. libGL.so is OpenGL. These files are Nvidia's own hardware accelerated OpenGL implementation.