I've just installed my main workstation at home with CentOS 5 ... this one has a nvidia video card (GeForce 6600) and i installed official nvidia drivers ... but after that i was not able to enable desktop-effects (compiz). You need to add something in your /etc/xorg.conf for compiz to work with nvidia drivers ... here they are (verify that they exist in your existing xorg.conf or add them) :

Section "Module"
Load "glx"
Load "extmod"
EndSection

Section "Device"
Identifier "Videocard0"
Driver "nvidia"
Option "AddARGBGLXVisuals" "True"
Option "DisableGLXRootClipping" "True"
EndSection

Section "DRI"
Group 0
Mode 0666
EndSection