I had this problem too when I tried out openSuse ( After I got it running, I switched back to Kubuntu), I had to manually create an Xorg.conf file to enable desktop effects.... specifically the video card section:
(note - this is for running dual monitor setup - so remove TwinView, TwinViewOrientation and MetaModes if you don't run two monitors.)
Code:
Section "Device"
Identifier "nVidia Corporation G71 [GeForce 7900 GT/GTO]"
Driver "nvidia"
BusID "PCI:1:0:0"
Option "RenderAccel" "true"
Option "SWcursor" "off"
Option "HWcursor" "on"
Option "CursorShadow" "on"
Option "TwinView" "on"
Option "TwinViewOrientation" "LeftOf"
Option "MetaModes" "1680x1050,1680x1050"
Option "AllowGLXWithComposite" "on"
Option "AddARGBGLXVisuals" "true"
EndSection
Also check that you have the correct driver for you video card, nvidia have two, one for newer cards, and a legacy one for older cards