Scenario #1: The US wins against Germany.
If that happens, the United States would not only advance, they'd win their group. That would also give them an easier matchup in their first knockout game, since they'd be playing the 2nd seed from Group H instead of the group winner.
Scenario #2: Tie with Germany.
A tie would also ensure that the US would advance. They'd be the #2 seed from Group G, with Germany winning the group, but they'd still be ensured of advancement.
Scenario #3: United States loses to Germany.
This is where it gets quite tricky.
If the U.S. loses by one goal to Germany and Ghana wins against Portugal by a margin of two or more, the U.S. will go home.
If the U.S. loses by two or more goals and Ghana wins (no matter by how much), the U.S. faces elimination.
In the unlikely (but not impossible) scenario that Portugal beats Ghana by five or more goals, the U.S. would be eliminated with any kind of loss.
If Germany beats the U.S. by five or more goals, only a draw between Portugal and Ghana can save the U.S.
If the United States loses by a margin of three goals or less and Portugal beats Ghana by the margin of one goal precisely, the U.S. still advances.
If Portugal beats Ghana by exactly one goal, the U.S. still advances as long as they lose by no more than three goals.
If Portugal beats Ghana exactly 1-0, the U.S. can lose by as much as four goals to Germany and still advance.
If Portugal beats Ghana by a margin of two goals precisely and the U.S. loses by more than three goals, the Americans will be knocked out of the tournament.
There are of course a few other scenarios, and Yahoo has the full chart on their site, in quite a fantastic format. Check it out there to see exactly what it will take for the United States to advance in the event that they lose today to Germany.