Right, this is what I have tried on my 8800 GTS.
Downclock both 2D/3D setting (together) using rivatuner
Setting 1 : Stock core/mem : 513/792mhz, fan speed at 80%
Setting 2 : Downclock core/mem : 300/400 mhz, fan speed at 50%
I'm using rivatuner launcher to either set it to setting 1 (using F9 key) and setting 2 (using F10 key).
This way, when I'm in GC idle mode, e.g. surfing, folding...., I'll just press F9 key and Setting 2 will be set.
If I wanna play games, I'll just press F10 key and Setting 1 will be set.
So far, the downclocking is stable. I can even play 3D games at full setting on that speed!!! No problems switching from setting 1 to 2 or vice versa....
The reason I'm not setting the 2D and 3D separately is because I need to find out whether there would be any problems if it the GC automatically rev up from 2D settings to 3D settings. Already posted in Nvidia forum by no good replies yet. Might just email customer support if I don't get any answers.....