Buy something that she doesn't have. Ask her what she wants. Most girls who own Barbies can rattle off a list of items they have to have. Decide how much you want to spend. Pick a price range you are comfortable with and look for a doll that matches it.
Kids know the difference.
That pink box means something to them. It's OK to buy dolls that aren't available as Barbies this way though, such as Baby-Sitter Club characters and other licensed characters. Look at what comes with the Barbies you are considering. Some come with lots of small accessories that will be easily lost unless your child is very careful. Understand that not all Barbies are made alike.