You'll need to insert the new module. As su do 'modprobe nvidia'. Don't forget to make the edits in your X config file. /etc/X11/XF86Config or xorg.conf depending on which you're using. Comment out with # in the driver section load "opengl" and load "dri", and make sure it has load "glx". In the device section for the video card change Driver "nv" or 'vesa" to Driver "nvidia"
I don't know where to autoload modules for Slack, so you'll have to find that yourself, unless a Slacker can shed a little light there
Hope that helps,