At 800 x 600 for the open-source Nexuiz game, both operating systems are running at roughly the same speed. With the same graphics driver in use, the OpenGL quality between them is also the same.
As the resolution increases, PC-BSD 8.2 begins to pull ahead of Ubuntu Linux.
At a modest resolution of 1400 x 1050, the Nexuiz Linux binary on PC-BSD was about 10% faster. The Nexuiz 64-bit binary is not any faster than the 32-bit build on either Linux or PC-BSD.
At the native panel resolution of 1920 x 1080, the Nexuiz Linux game runs 13% faster under FreeBSD/PC-BSD with the Linux KDO then it does when on Linux itself.