Xanax is a mild benzo and .5 mg is the standard dose. It usually takes effect about 20 minutes after you take it (possibly longer on a full stomach) and will last about 8 hours.
Beta Blockers are good for "stage fright" as they block the physical responses to adrenaline, however, they won't calm your mind like a benzo will. So, I'd say experiment with the Xanax and find a dose that works for you. Then take that dose about 45 minutes or so before your presentation.
Courage isnít about denying oneís vulnerability, but showing strength, despite it.