Research at the University of Alberta reveals that hand gestures, when used during conversations, may be an attempt by your brain to better express your story through language, rather than a representation of meaning. When I lived in Italy as a kid, I was enthralled by how expressive Italians become by reinforcing their conversation with hand gestures (great photo here).
As an adult, I give my clients the asinine advice that they should keep their hands still and to the side of their desk/microphone/podium. If they have to reinforce a point, they should make a gesture with a gently closed fist, the thumb crossing the bent forefinger towards the audience, not an accusatory and possibly unsettling pointed finger.
Here's a guide to Sicilian hand gestures - many of which can result in sudden physical pain dished out by the receptor (how's that? throwing out some communications theory for the homies!).
Or Neapolitan hand gestures (a helpful hint about the explanatory grid: it seems that "coming soon" really means "this gesture is so rude, I'm afraid my priest will read this and give me penance.")
A pretentious little tidbit on Hidatsa Native American gestures.
How about a general pointer on how to behave in cross-cultural situations?
And, in case you're a real geek, an academic discourse on Italian hand gestures from the sixteenth to eighteenth centuries, with plenty of academic references.
But maybe we should just leave the emphatic gestures to musicians: