Included with smxi is the graphics driver installer known as sgfxi. It can also be downloaded as a separate script. Like smxi, it must be run out of X desktop:
1. Ctrl-Alt-F1 will drop you out of X desktop to a command line prompt
2. Login as root
3. Type sgfxi
The sgfxi script automatically downloads and installs the kernel headers and every thing else it needs. This script downloads the binary graphic driver installers from either nvidia or ATI, prepares system, installs, then sets up xorg.conf, all in a fairly clean, reasonably intuitive way. Plus it updates itself so any new drivers released will be installed. Finally, sgfxi allows you to easily switch between proprietary non free drivers like ATI's fglrx and nVidia's nvidia driver and the free xorg drivers.
sgfxi can be run with numerous options. Enter sgfxi -h to see all the options and check the sgfxi Manual at the documentation links below.