The GeForce 8 support under Linux had started out quite problematic with a serious performance regression and other troubles, while through driver updates this is no longer an issue. However, these driver updates haven't affected earlier GeForce generations by much. From the seven drivers benchmarked with the GeForce 6600GT there are no performance improvements (or drops) to report.
Last year the new noteworthy features for the NVIDIA driver had included GLX_EXT_texture_from_pixmap support for Compiz/Beryl as well as improvements to the nvidia-settings panel, and the year before that there was Linux CoolBits and the introduction of Linux SLI (among other changes). However, this year most of the work done has surrounded the GeForce 8 series.
In our 2006 NVIDIA AYiR we had warned that 2007 would likely be the hardest year yet for NVIDIA on the Linux front and this has certainly panned out to be true. In addition to their initial GeForce 8 problems, they are now facing serious competition from ATI/AMD and their new Linux driver and new open-source strategy with open specifications. The developers behind the Nouveau driver are making steady progress and are working on an open-source Gallium3D component for NVIDIA graphics cards. Next year will be a critical time for NVIDIA on the Linux front as they determine how to respond to this increased competition and likely reaching the 200.xx Linux driver series. How they will respond, however, right now is anyone's guess.