There are these packages in unstable which you probably want to install:
xserver-xorg-video-nouveau The 2D DDX
http://packages.debian.org/unstable/libgl1-mesa-dri The 3D drivers, see MesaDrivers for their support status.
Other packages will automatically be installed as dependencies, such as
Newer versions of these packages are occasionally available in Debian's experimental repository.
If you run into problems, you may want to install various -dbg packages to debug them: