What is the long line at neck called? Do guys like them?

I don't think there is a single word for the actual neck region alone. What I do know is the area which makes up her neck, shoulders, back and upper chest, which she exposes by the neckline of her clothing is called the [i]Décolletage[/i] or, I think, the [i]décolleté[/i]. Other than this wide definition, it could only be said to be her 'neck tendon' which is supported by the muscles in her neck (and the upper spine, of course). :)