assuming you are still using the stock fan hub and ac fan in the stock shroud...
The stock viscous fan comes on at a temp decided by the oil pressure inside the fan hub, which in turn is controlled by little valves opening as the spring on the front of the fan hub turns as more hotter air from the radiator moves past it, once enough heat from the rad is moving over the little clock spring, it tightens up enough to twist the shaft which opens the oil valves. You can add more silicone oil to the hub (10-20ml) as this will increase the pressure of the oil and in turn make it come on at a earlier temp, but what temp exactly is guess work as to how much oil you add...... nissan decided on a certain volume of oil and that volume means it will direct drive off the water pump at a certain engine temp.
the other electric fan comes on as mentioned above, you can splice in wires run off a 12v feed and then tap and screw into your water return housing a thermo switch and you just buy what ever temp thermo switch you want.
or
just leave it and let it do what it does best, maybe just replace the fan bolted to the viscous hub with one of these
http://www.gktech.com/index.php/s13-s14-s15-replacement-engine-fan-60-more-air-flow.html