Actually the CA stands for Clean Air and if I remember correctly was only availabl in Cali. It seems that Cali has stricter emissions requirements and Nissan built this version of the sentra to fill that notch. Other than those changes I think its just a GXE.
It uses the 1.8L engine, but adds a 3rd catalytic converter, and a gas vapor lock system for the gas cap. Nissan claims that you can drive the CA GXE model for 10 miles and emit fewer pollutants than a typical car sitting in a parking lot not running. That is pretty cool.
The CA version makes 5 less HP and costs $500 more than a similarly equipped GXE model.
Just to add to this, the CA model was built specifically to use the specially formulated gas(low sulfer or something) that is only available in california. Fuel in other states could do damage to the engine if used for long periods of time.