CX310-66 is an upgrade for those who have already passed older version of SCJP. If you're already SCJP certified, then you can go for it. The content is the same as CX310-65, but there are less questions. If you are not certified yet, you must apply for CX310-65.
The prerequisites written at
Sun's homepage are wrong. It says "None", but it should be : "Successful completion of previous version of Sun Certified Programmer for
Java 2 Platform exam".