What will taking 300-400mg of trazodone do if I'm used to taking only 200mg?

That depends. Being unsure why you are trazodone, or what affects the medication is already having on you, it is difficult to predict what increasing your dose may do. Trazadone was originally released as an antidepressant. It typically required 300 to 450 mg daily to be effective for depression . It was never very popular as an antidepressant because of excess sedation in the morning at those doses.