When traveling to the U.S. everyone is required to complete a U.S. Customs and Border Protection Declaration Form 6059B. You may bring large sums of money with you in the form of cash, money order, or traveler's checks. There is no maximum limit, however, any amount exceeding $10,000 USD must be declared upon arrival on both the Form 6059B and FinCEN 105.
All forms must be filled in completely and truthfully. The penalties for inaccurate declaration and non-compliance can be severe including heavy fines and/or confiscation of funds.
For more information, refer to the U.S. Customs and Border Protection Agency web site.