The first variable is VIDEO_CARDS. This is used to set the video drivers that you intend to use and is usually based on the kind of video card you have. The most common settings are
nouveau for NVIDIA cards or
radeon for ATI cards. Both have actively developed, well-supported open-source drivers. If you have a newer AMD video card (Southern Islands and up), you may also try the new open-source
amdgpu driver. See the AMDGPU article for more details.