Beginning with Doom 3 on the Linux 3.6, 3.7, and 3.8 Git kernels for the GeForce 9800GT/9800GTX/GT220 graphics cards, there isn't a whole lot to see. The Nouveau DRM driver performance was unchanged between Linux 3.6 and 3.8 when running the id Tech 4 game on this open-source driver.
With the defunct Nexuiz open-source game, the performance was also unchanged for these mature NVIDIA GeForce graphics cards when comparing recent Linux kernel releases.
With the older OpenArena game, we're finally seeing some performance changes out of the Linux 3.8 kernel. For all three graphics cards, the performance was improved with the Linux 3.8 kernel. The improvement was most noticeable with the GeForce 9 graphics cards with the 9800GT OpenGL frame-rate going up by 5% and the GeForce 9800GTX going up by nearly 11%.