Splitting the power source between an arduino and a servo
If you have a 7V to 12V - 2A regulated supply, you can directly power your board using the 2.1mm connector. Do not go above 12V or you may destroy the voltage regulator on the board.

If you have a 5V to 6V - 2A regulated supply, you can supply your servo with this supply and rely on USB to supply the rest of the board (make sure you have a common ground for both the USB and the regulated supply).