The 2004/2005 controllers both have a PIC18F8520
The 2006/2007 controllers use a PIC18F8722
The difference is more memory (and some silicon errors).
Variable Space - PIC18F8520 2048 bytes + 1024 bytes EEPROM
Variable Space - PIC18F8722 3936 bytes + 1024 bytes EEPROM
Program Space- PIC18F8520 32K
Program Space- PIC18F8722 128K
(see
http://www.ifirobotics.com/rc.shtml#Specifications)