I'm currently working on some Mesa 11.3-dev + Linux 4.6 Git benchmarks comparing the GeForce GTX 900 line-up on the open-source driver code to that of the NVIDIA 364 beta driver series. Those results should be here in a few days while this article is a quick guide covering how to get all the necessary bits in place.
Before thinking of switching over to Nouveau for the latest generation NVIDIA GPUs, keep in mind this support is still very early. Additionally, Nouveau on the GM200 Maxwell GPUs currently exposes just OpenGL 3.3 (compared to GL 4.1 with Kepler on Nouveau or OpenGL 4.5 with the binary driver), there isn't yet any pstate re-clocking support to allow these discrete GPUs to perform adequately, etc. These pseudo steps are from an install I did on Ubuntu 16.04 but should likely also work with similar distributions/releases.
Ubuntu 16.04 is shipping with Mesa 11.1 currently but Mesa 11.2 is expected to be added soon. However, for best support, you'll want Mesa Git. Switching over to Mesa Git (currently 11.3-devel) is as easy as using the Oibaf PPA. Add that PPA and then do a dist-upgrade on the system.
The firmware binary blobs needed for the Maxwell support are currently in Mesa Git. Fetch the latest firmware blobs from linux-firmware.git and install them (sudo make install).
You'll lastly need the Linux 4.6 kernel. The DRM-Next code containing the necessary updates to Nouveau DRM landed a few days ago so until the RCs begin to appear, just grab the latest Ubuntu Mainline Kernel PPA packages.
Once getting the latest Mesa, fetching the latest firmware, and installing the Linux 4.6 kernel (along with updating the initramfs if installing the firmware after the kernel upgrade), you should be good to go for then having Nouveau OpenGL acceleration working with the GeForce GTX 900 series line-ups. I did run into one small issue is that on a reboot at first I was getting strange Nouveau errors in the dmesg, but after then turning off the system and back on rather than just a reboot, everything has seemed to work out fine. I'm currently running some benchmarks of the entire GTX 900 line-up and will have them on Phoronix in the next few days.