Definitely Liberty City and definitely New York City; the folks at Rockstar have always modeled Liberty City after New York City (much like San Andreas is supposed to be Los Angeles, or one of the surrounding cities in the San Fernando Valley). With the improved graphics for GTA IV, things like the Statue of Liberty and Empire State Building will be more visible. No word as to what Rockstar will call the Statue and Building, since I doubt they can use the real life names.