As mentioned countless times on Phoronix, the main limitation of Nouveau's "Fermi" and "Kepler" GPU support is that there's no support yet for re-clocking the core, shader, and memory clocks. As a result, the graphics cards are left running at whatever frequencies they were set to by the NVIDIA video BIOS. In many cases, these vBIOS-set boot frequencies are extremely low. Pre-Fermi GPUs on Nouveau have experimental support for re-clocking via setting a special kernel module parameter and writing some sysfs values out, but there isn't yet this capability for the recent NVIDIA GPUs and it will not be until Linux 3.13 or even 3.14 when this support becomes experimental for GeForce 400/500/600 series.
While the GPU is left in a crippled state, if you boot the GeForce GTX 650 "out of the box" on Ubuntu 13.10 you do have 2D and 3D acceleration via the Linux 3.11 kernel and Mesa 9.2.1. The support is decent and handling the latest NVIDIA hardware doesn't require messing with GLAMOR, any LLVM back-end, or other requirements like there is with the latest AMD hardware on the "RadeonSI" driver.
For showing how the open-source Nouveau driver performance compares to the NVIDIA driver, I benchmarked Linux 3.11 + Mesa 9.2 against NVIDIA's 319.60 binary driver as packaged for Ubuntu 13.10 x86_64.
The benchmark results can be found on OpenBenchmarking.org via 1310255-SO-GEFORCEGT21. Of course, you can compare your own system's GPU/driver performance against this data by installing the Phoronix Test Suite and running phoronix-test-suite benchmark 1310255-SO-GEFORCEGT21.
When looking at all of the results on OpenBenchmarking.org you will see the Linux OpenGL performance with Nouveau on the GTX 650 is generally less than 20% the speed of the official NVIDIA Linux driver.
Now go see the rest of the results on OpenBenchmarking.org. In the next few days I'll also have Nouveau vs. NVIDIA results for a greater selection of hardware, also including the Linux 3.12 + Mesa 10.0 development code as a third data point for each GPU.
As soon as re-clocking comes for GeForce 400/500/600 GPUs (and stabilized), the Nouveau performance should be much more competitive to the NVIDIA binary driver. In the GPUs where I have it working, Nouveau performs much better against NVIDIA's binary blob.