I'm using 3 ATmega328P processors for the prototype circuitry. The first is used to communicate with the R/C receiver and filter the results. The second is used to communicate and process the sensor data from the accelerometers, gyros, and compass. Those talk to the main processor over TWI which combines the receiver and sensor data to control the motors. The main processor also sends out telemetry data using an XBee serial connection as well as monitor the battery voltage.