But how far beyond? Jay Manifold discusses the potential limit of civilization, and links to another article on the same topic titled "The Physics of Extra-Terrestrial Civilization". Lots of somewhat-reasonable speculation, that's fun to think about.
Personally, I'll be quite surprised if we ever find extra-terrestrial intelligence, and I'll be only a little less surprised if we find ET life of any sort (after all, if there's any life, then there's probably intelligent life somewhere). Rather than waste time searching for such things, I think we should concentrate on getting as many people off the planet ASAP.