Can't quite see why you would want to turn it off.. I've the same graphics card on a 2.53ghz p4 with 512mb and with all the scenery / complexity sliders hard over to full, and all the other effect boxes ticked, and the absolute slowest frame rate I have seen is 35 FPS, normal running 60 to 80, up to well over 100. This with the latest Nvidia drivers.
Tim