To answer your question, Hannibal: In my opinion, greed put Rome at the top, via the Republic. The Empire put restrictions on that greed, allowing Rome's vast empire to survive.
Felix, with all due respect I have to disagree. Rome's republic was an oligarchy. As with America, it had democratic forms, but all oligarchies are based on that always reliable aspect of human nature: greed. It's also a reliable form of government, where the rich band together and protect their mutual interests. Every lasting government has been based on this principle. Pompeius and Caesar were products of their culture, the way had been paved already by Marius and Sulla. I'm not saying that the Republic wouldn't have worked, but there were other men who'd have eventually seized power or contested the system. Or had already tried, like Catiline during Cicero's consulship. Human nature is like a law of physics. Greed would have caused endless looting of the provinces, and endless new border wars that would have ultimately destroyed the empire. Marxism didn't work either, for exactly the same reasons.
On that note, though - we all have to remember that Republic and Empire are, in part, classifications applied by modern historians. In his own time, Augustus was revered for restoring the Republic, though some of his contemporary critics denounced him. But nonetheless, Imperial forms lasted well into the Republic. Diocletion was the first emperor to act and dress like a king, at least by Roman standards. (He was also the only emperor to retire. And later come back as a consultant!) The earlier Imperial period was still the Republic. It just had one additional Magistrate, the princeps. But even this was decided on and ratified through the time-honored Republican legislative processes.
(Edit: Augur, I was writing while you posted. I didn't mean to come off as reiterating your excellent post.)