NVIDIA's 352 Linux Driver Against Linux 4.1 With Nouveau Gallium3D
The GTX 650 and GTX 680 were playable with Team Fortress 2 on Linux for Nouveau while the performance was significantly better on the proprietary driver.
As the GTX 650 shows, when the GPU and video memory are fully re-clocked the performance on Nouveau is decent for being an open-source, community-based driver, but the official driver reigns supreme.