Get this driver from
Nvidia downloaded to your /home/<username> directory. While it's downloading, go to the
README - Text Version link on that page. The instructions are fairly easy to follow. This driver installs very easy. Just check to be sure you meet the software requirements before you start the install. NB: You must be online when you install the driver, as it will need to go to the Nvidia site. After you install it, edit your /etc/X11/XF86Config file per the Nvidia instructions. I am running Slack 9.1 so that file I mentioned may be slightly different on your version of Slack.
I suggest the older version of the Nvidia driver (NVIDIA-Linux-x86-1.0-1.0-4496-pkg2.run), and not the latest one, because many people have had problems with the new one (NVIDIA-Linux-x86-1.0-5328-pkg1.run).