If one forgets about libraries and clients, a whole X stack boils down to: the server itself, input and video drivers, libdrm, and mesa.
To keep things simple, the idea is to backport all of those to squeeze through squeeze-backports, along with some of the additional packages which might be involved (like libxfont or x11proto-fixes-dev).
The backports were prepared so that it’s possible to upgrade the input-all and video-all meta packages on amd64 and i386. If specific drivers (maintained by the X Strike Force) are wanted, requesting them on the debian-backports@ / debian-x@ mailing lists should do the trick. For other drivers, please contact the relevant package maintainers.
The usual backports instructions apply. But let’s gather everything in a single place.
Add that to your sources.list:
deb http://backports.debian.org/debian-backports squeeze-backports main
Update your cache:
If you’re interested in just upgrading the usual mesa packages:
apt-get install -t squeeze-backports libgl1-mesa-dri libgl1-mesa-glx
If you want the whole stack, we should distinguish between two cases.
In the usual case, both the xserver-xorg-input-all and xserver-xorg-video-all meta packages were installed, and pulled a lot of packages, which should work for most users. Specifying a few packages to upgrade should pull everything from squeeze-backports.
apt-get install -t squeeze-backports xorg xserver-xorg xserver-xorg-core xserver-xorg-input-all xserver-xorg-video-all apt-get install -t squeeze-backports libgl1-mesa-dri libgl1-mesa-glx
In case only the needed packages were installed, like the synaptics input driver and the intel video driver, the following should be sufficient:
apt-get install -t squeeze-backports xorg xserver-xorg xserver-xorg-core xserver-xorg-input-synaptics xserver-xorg-video-intel apt-get install -t squeeze-backports libgl1-mesa-dri libgl1-mesa-glx
It is probably a very good idea to install the Linux kernel from squeeze-backports as well. It is even required for the nouveau video driver.