Which living American will be remembered 500 years from now?  

It's impossible to say what kind of society there will be in 500 years time. America itself might not exist as a coherent nation by that time. Heresy I know, but nations have a life span just as people do, and 500 years is a long time, longer that many civilizations have lasted for to say nothing of individual nations. To try and give some sort of answer to your question though, we have to think of the people who lived 500 years ago and who is well known now and why. It was outstanding rulers or people who did outstanding things that in some way have relevance to the people who live now. Maybe if the people of 500 years from now are living in a futuristic technological world then maybe they'll most remember Bill Gates. Personally though, I can't think of any American president who deserves the description "great"