What is the name of the spec that describes base voltage for minimum resistance between the collector and emitter? That's all I want to know.
Perhaps you missed it but your question was precisely answered in reply #8. You want a "logic level" N-channel FET; that IS the spec that you are looking for. That means the transistor is turned "fully on" with minimal resistance between drain and source with 5volt to the gate . If you look at the general specs for the FET's linked in reply #1, you will see that they are indeed labeled as "logic level".
As recommended in previous replies, FET's are generally preferable to Bipolar Junction Transistors for your application. But if you are unfamiliar with N-channel FETs, simply consider the Drain of the FET similar to the Collector of your NPN BJT, the Gate of FETs the same as the Base of your BJT, and the Source of the FET similar to the emitter of your BJT.
So typically your circuit would be something like this: connect your Arduino output pin to the gate of the fet. connect the battery " + " to the " + " of the motor. connect the motor "-" to the drain of the fet. connect the source of the fet to both the battery "-" and the Arduino ground. just like the diagram here:
But for your application I am concerned that you will need to reverse the motor in order to move the arm in two directions, the above circuit will only allow the motor to turn one direction. Forward and reverse will require more complicated circuitry, usually involving(you will need to do a search) H-bridges. You also need to search about "flyback" diodes, which are typically added to the above circuit in parallel with the motor to prevent damaging voltage spikes.
edited......here is another good link I just found that touches on the "flyback diode" and seems well put together