Well by definition an RC servo has the power electronics built into it already since the radio receiver obviously can't drive the DC motor inside the servo directly.
So by this (dare I say infallible?) logic, there must be a motor controller that responds to the same type of signals the servos do from the same radio receiver, otherwise how would everything work?
So, if this is the case, and it is a real RC car (with "real" swappable parts), then the radio receiver outputs PWM signals and the servo reads them and interprets them as a position and it's power electronics inside the box do the rest of the job to actually drive the motor. It is the same for the motor driver, except that it interprets the signal as a speed.
So the answer to your question is most likely NOTHING since you make it sound like the car came with servos, motors, motor drivers (ie. the modules) and was all ready to run. Just run a PWM line to each servo and motor driver (and provude them power from their non-logic power source of course). If it was not all ready to run though, you might need to get another one of the "modules", be it a servo or motor driver. You don't have to build it unless you want to control it in some other way (like if you didn't want the 8051 to simulate the signals of a radio receiver). But otherwise, just buy one- much cheaper and faster and easier.
Easy as apple pie.
EDIT: By real RC car vs toy RC car, I am asking if it is hobby grade or not.