What are the differences between the NXT and the RCX?
NXT
RCX




3 motor and 4 sensor ports
3 motor and 3 sensor ports


Has encoders integrated in its motors so that they can be used as servos
Would need an additional rotation sensor on a sensor port to use as servos


Uses RJ12 connectors with the latch offset to one side
Uses regular 9V connectors (2x2 with metal in studs)


Uses a direct USB cable or Bluetooth for connection
Uses infrared to communicate with PC via an IR tower connected by a serial (1.0, 1.1) or USB (2.0)


Graphical 100X64 matrix display
Small numerical display with many limits


Technic oriented - uses technic pieces (no studs)
System oriented (studs) with small amount of technic


Uses graphical programming language with many features1
Uses a limited graphical programming language (apparently can't even run on XP)


Can play 8Khz wav files
Can play simple tones


Uses 6 AA batteries very quickly
Uses batteries more slowly


256 KB of flash memory that can hold a max. of 64 files of varying types
32 KB of RAM that can store up to only 5 programs





The original 1.0 version of RCX Brick had a 10v connection plug, but this was dropped from the later versions included in the Robotics Invention System 1.5. The NXT can use a rechargeable battery in place of the six AA batteries, and the rechargeable batteries also have a 10v transformer plug.

RCX retains its memory (including firmware) for a very limited time when not powered, just enough to change batteries. NXT uses flash memory and keeps it all when not powered.


1: Note that even powerful computers have trouble running big programs, and can easily mess them up. There exist many third-party systems for both RCX and NXT. Full list on Wikipedia.