When running Nexuiz at 800 x 600, there is a very significant performance boost when switching to the latest kernel code that will eventually be called the Linux 2.6.39 kernel. The 8500GT is 48% faster, 9500GT is 54% faster, 9800GT ends up being 57% faster, and the GT 220 is 43% faster. It is simply from upgrading the kernel to take advantage of KMS/DRI2 page-flipping and other improvements.
As you can see, the latest Nouveau driver is now close to running at the same speed as the NVIDIA driver for the low-end ASICs, but when the resolution is increased, there's even more competition between the two drivers. With the higher-end GeForce 9800GT, the proprietary driver is still 2.7x faster than Nouveau.
With Nexuiz at 1280 x 1024, the delta between the latest Nouveau driver and the NVIDIA driver is even closer and for the 8500GT/9500GT is at the same speed as the blob.
Lastly, for Nexuiz, when hitting 1920 x 1080, which is the native resolution of the display and the running xorg-server, the Nouveau driver does its best. The percentage improvement between the 2.6.38 and 2.6.39 kernel code is not as great as the lower resolutions, but it is still very evident and it's all that it takes for the low-end GPUs to battle the binary driver.
With Nexuiz on high quality settings (including GLSL shaders, etc) as used by the Phoronix Test Suite, the 8500GT and 9500GT were slightly faster than the proprietary driver while the newer GeForce GT 220 was just two frames per second slower. The higher-end GeForce 9800GT still remained a great deal behind.