Interface 8051 DC motor using a DAC
Yes, if you intend to try driving the motor with a varying voltage to control it's speed then you would need a motor driver amplifier. 

A much better way to work this problem would be to simply power the motor from a 3->6V supply and use a MOSFET transistor rated for "more than a few amps" as a switch to permit turning the motor on and off at a very high rate. 

Use the 8051 with a timer or software to generate the very high rate at which the MODFET is turned on and off. Now when you generate such waveform vary the amount of time that the motor is off versus on. This will change the speed of the motor!! This technique is called PWM (pulse width modulation). 

When you begin to research PWM techniques to control motor speed you will also come to learn how to connect such driver to the 8051 and at what types of frequency you should be using for the PWM waveform.

Have fun!