Probable starting eleven:

USA:
Howard, Cherundolo, DeMerit, Onyewu, Bocanegra; Donovan, M. Bradley, Edu, Dempsey; Altidore, Findley

Ghana:
Kingson, Pantsil, John Mensah, Addy, Sarpei; Annan, KP Boateng; Tagoe, Adiyiah, Ayew; Gyan

I think USA have better team. They played in great style and won the group C before the big favorite England. On contrary Ghana didn't impress me a lot. They were lucky to qualify for knock out stage after Serbia failed to win against Australia. Ghana coach Milutin Rajevac played with only one striker (Gyan) and Ghana scored only two goals so far, one of them from a penalty shoot against Serbia. here is the results:

June 23: Ghana 0-1 Germany (World Cup)
June 19: Ghana 1-1 Australia (World Cup)
June 13: Serbia 0-1 Ghana (World Cup) 

On contrary, US coach Bob Bradley are favoring attacking football. Altindore and Findley are two very good goalscorers and I expect to be even more efficient in knock out stage. So far USA scored on every game:

June 23: USA 1-0 Algeria (World Cup)
June 18: Slovenia 2-2 USA (World Cup)
June 12: England 1-1 USA (World Cup)

I think USA is better team. They played an excellent game against Algeria, creating chance after chance. If they continue that way I am sure Ghana will not have any chance today.