Can Arduino tell C# or any other program what to d
The Arduino can send data to the serial port, which the C# application can read. It is up to you to define commands for the Arduino to send, and to write the code to make the C# application respond in appropriate ways.
If so, can those commands be made into graphic?
I have a pretty neat 3D graphics application written in C#. Has nothing to do with Arduino, and it's company proprietary so I can't share it. But, C# can do graphics. If you send sensor data to the application, C# could create a plot of it.
what is the maximum amount of FLOPS that Arduino can put out?
The Arduino executes 16,000,000 instructions per second.