With the GeForce 600 "Kepler" graphics cards now working on Nouveau in an out-of-the-box manner with OpenGL acceleration, here's a brief five-way NVIDIA GeForce graphics card comparison when using the Nouveau open-source driver with the Linux 3.8 kernel and Mesa 9.1-devel.
Nouveau with the Linux 3.8 kernel can be faster as earlier Phoronix benchmarks have shown plus the Kepler support is now in better shape, although it's still very slow and lacks re-clocking support. Other recently published benchmarks show that Nouveau is still no competitive threat to NVIDIA's binary blob.
In this article are benchmarks of the NVIDIA GeForce GTX 680, GeForce GTX 460, 9800GTX, 9600GSO, and 8400GS graphics cards when using the newest Nouveau Linux driver code. This is just an assortment of low and high-end graphics cards from multiple generations to see how the NVIDIA hardware is working out on Linux 3.8 + Mesa 9.1-devel.
The software stack, along with the Ubuntu 13.04 base operating system, stayed the same aside from swapping out the graphics cards. All of the graphics cards were running at their stock speeds as handled by the Nouveau driver at boot time -- unfortunately there is still no viable mainline re-clocking support.