Computer controlled potential difference across wires
The total resistance for 200 mA at 20V is 100 Ohm. The wires should be quite long or thin. 

But as answer: I guess you want to make some kind of heating. If you plan PWM control, high enough switching frequency should quarantee the temperature of the wire stays in certain range, no matter the current is alternating 
being part of the time 0 mA and the rest of the time 200 mA. Heat capacity and heat transfer calculations are medium complex and they are impossible without knowing better your actual system.

The guess can be wrong. If the current should stay in certain DC value you can well have an inductor, switching transistor and a flyback diode. Have high enough inductance to keep L/R much bigger than the switching period. You can find the needed inductance by simulating if you cannot do elementary time constant calculations. the Starting point is to decide how much the current is allowed to swing.