Even when the video cards are recognized, the screen can be unstable, and 3D support is not effective. It is desirable to install nVidia drivers (when you have an nVidia card).
Download a driver for your own card from http://www.nvidia.com/content/drivers/drivers.asp
To turn off X, type
# /sbin/init 3
and log in as root. Move to the directory where the driver is put, then type
# sh ./NVIDIA-Linux-x86-1.0-7664-pkg1.run
and the driver is installed. Then, type
# cp /etc/X11/xorg.conf /etc/X11/xorg.conf.org
and edit /etc/X11/xorg.conf to rewrite Driver "nv" as Driver "nvidia", and remove Load "dri". Make sure that the line Load "glx" exists, and if the line Load "GLcore" is there, remove it. Type
# /sbin/init 5
and X starts again. If you see the logo of NVidia, the driver is successfully installed.
Each time you update the kernel, you need to repeat the installation of the driver.
(information of the nVidia on Linux: NVIDIA Linux Forum