Oh, and I'm going to be controlling this motor with PWM.
PWM will do the trick! ...Wth a transistor or MOSFET to "boost" the voltage & current... You can't run a motor directly from the Arduino output.
A resistor will work, but it will have to dissipate power (heat) so you have to have the a big enough resistor and it's inefficient. And, you'll probably have to try a few different resistor values to determine what value you need.
Ie, how would it affect the speed and torque?
Torque, speed, and load are related. Often in an "unpredictable" way... For example, if you limit the current to a fan-motor it will slow-down. But if you remove the fan, there is less load and the motor will run faster. If you want to precisely control the speed of a DC motor, you need some sort of RPM sensor.