1. Open low-level fan control tab.
2. Configure desired fan settings for 3D mode (e.g. fixed 100% duty cycle) and save them to low-level fan profile, e.g. "3D fan settings".
3. Configure desired fan settings for 2D mode (e.g. fixed 50% duty cycle) and save them to low-level fan profile, e.g. "2D fan settings".
4. Open launcher tab and add new regular item, e.g. "apply 3D fan settings".
5. Tick "Associated fan profile" and select low-level fan profile type.
6. Select desired low-level fan profile, e.g. "3D fan settings" and finalize adding new
launcher item by pressing OK button.
7. Open launcher tab and add new regular item, e.g. "apply 2D fan settings".
8. Tick "Associated fan profile" and select low-level fan profile type.
9. Select desired low-level fan profile, e.g. "2D fan settings" and finalize adding new
launcher item by pressing OK button
10. Open low-level hardware monitoring module and right click "Core clock" graph, then select "Setup" from the popup menu. Take a note that we'll define threshold on core clock for detecting switching between 2D and 3D modes.
11. Add new threshold, e.g. "3D mode detection threshold" and define threshold value. Considering that default 2D core clock for 7800GTX is below 300MHz and default 3D core clock is above 400MHz, you may set the threshold value to 300MHz.
12. Configure threshold to launch "apply 3D fan settings" item on upward crossing. This will cause RivaTuner's hardware monitoring module to apply it anytime when clock is changed from <300MHz to >=300MHz (i.e. anytime when 3D application is started).
13. Configure threshold to launch "apply 2D fan settings" item on downward crossing. This will cause RivaTuner's hardware monitoring module to apply it anytime when clock is changed from >300MHz to <=300MHz (i.e. anytime when 3D application is closed).
source:
http://forums.guru3d.com/showthread.php?t=162506
Just in case you were too lazy to click on the link, more direct this way