You should use something like Redhat, or Suse that make this easy for you...
Just kidding!
First time installers of slackware will more than likely not be able to run X without first configuring it. I prefer to suggest running xf86config and while doing so you can tell it to run at 16x12 mode. 1 major hurdle I'd suggest first is installing your nvidia drivers for your card. Then, run xf86config and then edit the /etc/X11/XF86Config file according to the Readme for your driver, and THEN startx (also, in slackware, to make your life easy, use xwmconfig to choose a window manager for the first time).
Cool