I think I'm experiencing the same problem with my 7300 card.
I've got a PC (running a recent RELENG_7 kernel and world, xorg-server 1.5.3_8,1, KDE 4.2.1, nvidia-driver-180.29) and I have three NVIDIA graphics cards available:
- GeForce 6200 TurboCache 128MB (of on-board memory, that is)
- GeForce 7300LE 256MB
- GeForce 8500GT 512MB
If I plug in the 8500 and start KDE, desktop effects work properly. Likewise with the 6200. But if I reboot and plug in the 7300 (without changing any software settings), KDE won't enable desktop effects because kwin gives the error: "Compositing self-check failed, disabling compositing." Ticking the "Disable functionality checks" tickbox in the Desktop Effects control panel causes the screen to partially repaint then freeze for ten seconds before KDE gives up and reverts to the previous settings.
I've also tried running the 7300 with NVIDIA drivers 180.44 and the 173.14.12, but had the same problem.
I'm now wondering whether there's a Linux live CD with KDE 4.2 and the NVIDIA driver that I could try with my 7300 to see whether that works.