Quote Originally Posted by waxwing
And how about the hyphen? When should it be used?
First of all, this is a dash, not a hyphen .

More to the point, there a general, broad rule that covers use of dashes in such sentences: the dash signifies a shift of intonation or a pause. It is actually the writer's choice, just like changing or not changing intonation for emphasis is the speaker's choice. The same sequence of words can be pronounced smoothly, with even intonation and speed, or with a pause introduced for emphasis.

"Девочка сильная" (just a simple statement)

"Девочки слабые. Эта девочка — сильная" (There is a short pause after "девочка").

Please note, that if both the subject and the predicate of a sentance are nounse, the dash MUST be used in most cases: "Девочка — спортсменка".

You can also check this thread and download the Rosental's handbook. It is a classic, I've had it for years, and it is definitely worth having. There is a whole section about the use of dash ("тире") in various situations.