Well you would need some good torque motors, that is if you don't want to use servos. Or you could also use hydraulic pistons to move the arm.
Servos are good to start with, especially with a kit but for better stronger applications worm gears and hydraulics are the way to go.
One other thing, servos can not run off the 5 volts from the arduino, so you will need an external power supply that can output about 6 volts and decent current ~1 - 2 amps.