Why would you want to unplug the fan? They hardly make any noise (most power supply's make more noise than a GPU fan).
As for if you could, no on the GF3, not sure on the Radeon. Geforce cards tend to run hotter than the Radeons, and need the extra airflow. The Radeon fans are so small, that they might be able to be unplugged without too much of an issue.
I want to be your Opteron... Why don't you call my name - Peter Gabriel?