But, in your Arduino code, you are only reading and storing 3 characters in the Command array so this statement:
rightSpeed = 10*Command + Command;is always going to have problems. Your loop needs to be:
for (int i = 0; i<4; i++)
Also, if Processing sends the data as an ASCII string, you need to subtract 48 from each character before trying to convert it to decimal. You have a "- 48" in the code but it is commented out.
And Serial.flush() doesn't do anything in the Arduino code. This only flushes the output buffer (in IDE V1.0.1) and will have no effect.