At what rate should you gain muscle weight?

As I try to get in shape and lose fat, my scale is all over the place. I'm often sore after workouts, and I understand this is due to muscle gain/rejuvenation. When I weigh in, sometimes I plateau, or even gain, even though I'm on a strict diet and do moderate exercise. So how much of this weight is really muscle? How fast is it appropriate to gain muscle?