Is there a seperate executable I need to download to run the game in 64 bit mode?
No. the game automatically checks this and runs with Large Address aware if you are running 64 bit. Don't believe me check to see if you have pedestrains when you zoom in close, this option is disabled for 32 bit systems.
But they haven't released a 64 bit .exe which I wouldn't mind seeing this with some extra content. Last I heard it was going to be an expansion type thing they may be planning.