Please focus on the moment and not the greatness.
Unlike great athletes, entertainers or businessmen, great presidencies are driven by responses to external events. There is no chance to manufacture a knockout, merger or hit single that transcends time. You take what the position gives you. And if you're human, you hope and pray it's not much. If the past 6 years have given us anything, it's a case study in the folly of attempting to manufacture greatness.
President Bush desperately wanted to be a war president in the mold of Washington, Lincoln and FDR, so he manufactured a war, ostensibly to feed that desire. But he was confused. Revolution, emancipation and democracy were watershed moments to which greatness reluctantly rose, not events manufactured with ersatz greatness in mind. As such he leaves office en route to a potential worst ever label all but ensured for the existence of the union.
What's weird is that unlike Washington, Lincoln and FDR who each struggled with recalcitrant citizenry, his watershed moment - wished upon no one - actually saw him gain the full support of the nation in the blink of an eye. What's weirder is that had he focused more on the moment and less on legacy. More on effectively dealing with this new threat and less on 2050 history textbooks, he would be leaving, destined for greatness.
So again, I implore you to focus on your moment or moments as it may prove. Arab Radicalism? Depression?? TBD??? Whatever it is focus on it. Your legacy, like Washington, Lincoln and FDR will write itself.