This question is a little vague, but maybe I can provide some info that will allow you to improve the quality of your post.
I'm assuming that you have the OpenCV portion working, and you indeed get X/Y position values. If you have OpenCV working, then I assume you can also link to its binaries from your compiler. For example, you can add references to the .NET build of OpenCV (emgucv) in Visual Studio. Your own app will then be able to get the X/Y values.
At this point, you now need to control your servo motor. You could interface with it in several ways. One way is to get a dedicated servo motor driver / controller that communicates with your PC over a variety of physical formats -- RS232, CAN, USB, Ethernet, Ethercat, are some examples. If you have one of these, there is most likely a library that you can also link your application to to control the motor.
Another option is to use an Arduino, a proper motor shield, and then connect your Arduino to your PC via the USB connection. This becomes a virtual COM port and you'll then need to implement your own RS-232 communication protocol to send commands to the Arduino. The code running on the Arduino would then parse these incoming commands, and then control the motors via the motor shield.
If you can make your question more specific, you will likely get even more help from people here.
If the Arduino does not reset when the servos are not plugged in, then likely what's happening is that the servos draw a lot of current and so the battery voltage drops enough that the Arduino resets.
Try using two separate battery packs: one to power the Arduino, and the other to power the motors (make sure to connect the grounds of the two battery packs together).
Best Answer
You didn't mention how much current you require. Here is a quick guide --
For most switching applications the important parameters are the voltage rating (BVdss), the maximum drain current (Id(on)) and the gate turn-on voltage.
For a 6V battery you want a breakdown voltage at least 6V. Make this a bit higher in case switching produces transient voltages. Since the majority of FETs have voltages of 20V or greater this should not be a problem. Choose a 20V or 30V FET.
Choose a maximum drain current above what the servo requires. The maximum drain current is usually limited by the thermal performance of the system not the device. How much current do you need? How large a device can you use? Do you have room for a heatsink?
To use the FET as a switch in a 3.3V system you want a logic level device. This will insure that the device is fully on (lowest on resistance) at 3.3V levels.
For circuitry I will usually put a pull-down resistor on the gate so that the gate is never floating. For some applications I will place a zener diode across the gate for transient protection.