1) It is a good sign. It means your case is in process. If you can pass through the interview, yes you will receive your GC very soon.
2) all the cases to be interviewed will be transfered to local NBC.
3) you will find answer when you receive the letter for interview.
4) check this. you will learn more about NBC and possible reason for your interview.
http://www.trackitt.com/discussion-forums/i485-1/4343221/last-page/