I recently upgraded my D800 to Ubuntu Hardy. Things seem to be working fine. Looking through the packages available I saw that I could get sensors-applet to monitor internal temperature sensors. This showed that my GPU was running near 75 degrees C.The nvidia-settings tool showed I was running at Performance Level 2 and appeared to be stuck there. I searched a bit and found other people looking for the same info and got a few hints. After some fooling around I found the following commands would cool things off. They may reduce performance but I don't need it.
nvidia-settings --assign="GPUOverclockingState=1" nvidia-settings --assign="GPU2DClockFreqs=100,230" nvidia-settings --assign="GPU2DClockFreqs=100,230"
Yes, I know that I'm setting the 2D Clock Freqs twice. The first time seems to move it to Performance Level 1 and the second time drops it to level 0 where I want to be. I got the numbers from the nvidia-settings on the Power Mizer tab.
I put those commands in a script and run it using the Sessions facility I found at System->Preferences->Sessions.