Major Festivals are great for media exposure, but they reach a really limited audience. Sundance is predominately film industry professionals and wannabes; what about the real ticket buying people? If someone hears about your film and they can't attend the festival, how will they get to see it?
With your audience's interest piqued, is it a good time to get your film online soon after the festival screening? What method will best serve your film: streaming, ad-supported, pay per download? There are many variations on this, but the point is you need to have it figured out before your screening if you are going to take advantage of it. And you need to have some way to let people know.
Some festivals, like Slamdance, are doing this directly themselves, and I think that's a great idea.