I have an Ubuntu 9.04 PC with an nVidia graphics system running at 1920x1080 outputting through an HDMI port. This all works fine until I enable the visual effects from the System > Preferences > Appearance... dialog. Then, the screen update freezes and the screen updates, when they happen, tend to contain bits of other windows / corruption. The system is still running, albeit very sluggishly, so I can switch the effect off. I'm using the restricted nVidia drivers (there may be a newer driver, but I've tried a few versions and they all exhibit the same problem).
Any ideas what's going on and how to fix it?
From what I remember (the PC is at home, I'm writing this at the office), it's using this motherboard which has an nVidia GeForce 9400 on it.