The NVIDIA 177.76 Linux driver was released a week ago with a few changes from the earlier betas that made it out of NVIDIA's Santa Clara campus roughly a month ago. Now today NVIDIA has issued another Linux beta driver, which is at version 177.78. The only changes since the NVIDIA 177.76 release is fixing a performance regression within the GtkPerf GTK benchmark, using /sys/class/power_supply for determining the power-state within the driver, and fixing corruption when using SLI in SFR mode with OpenGL-based composite managers. The change-log and download links can be found within the NvNews forums...
It may, because last driver release (177.76) they mentioned Xrender improvements, but they were not noticeable because of a regression on that driver, which is now gone.
I will test KDE 4 now (currently on Gnome)
EDIT: Doesn't seem much faster on KDE4. Not slow at all, but not 100% smooth. But I've never had too many problems with KDE4, and I don't use it much anyway, so I probably wouldn't notice the problems so much. But I can say that gnome is 100% smooth and KDE4 isn't.