More googling gave me this, taken from a poster on Guru3D forums:
"Overdrive has to do with making the pixels go on and off faster i think to eliminate ghosting. If you don't see any ghosting now, no need to change it. (open paint, make a big black image and move your pointer around. If you don't see any ghost like trails no need to change it)
In essence, it's what makes your monitor run 2ms as opposed to 5ms."