Shrestha456...since you have valid visa until 2008,you don't have to go Consular Office again..Same Visa will be valid for reentry but since you will be going Nepal in December and probably coming back sometime in January...your I-20 needs signature from your International advisor not earlier than January 2006...if it was signed earlier than a year ago,please ask your advisor to issue new I-20..it is just matter of couple of minutes work.. Enjoy your holidays...