Put this in your xorg.conf:
Option "PixmapCacheSize" "200000"
if you have a laptop
put this in /etc/modprobe.conf (or create a file in /etc/modprobe.d/)
options nvidia NVreg_RegistryDwords="PerfLevelSrc=0x2222"
options nvidia NVreg_Mobile=1
reboot (or from console (without X started): rmmod nvidia;modprobe nvidia)
run: nvidia-settings -a InitialPixmapPlacement=2 -a GlyphCache=1
I have NVIDIA 8600M GT. Kde now works pretty good.
If you don't have an laptop I suggest you to buy ATI !!!