The top three finishers in the short program have control over their own destiny in the long. If they win the long, then they win the gold.
The skater in 4th after the short needs some help to win. If Hughes, in 4th after short, wins the long, and Kwan, in first, finishes second in the long, then Kwan still wins the gold. Interestingly enough, one of Kwan's world championships was won with this scenario. She was in 4th after the short, won the long, and the leader after the short finished third in the long.
The bottom line to you question: If Hughes finishes first in the long, and Kwan finishes third, then Hughes wins the gold.
Technically, a skater in even lower standings after the short can win the gold, but the results of the long have to break his or her way, and I wouldn't look for that to happen tonight.