When traveling to China from the US what currency is recommended, $100, $50, new bills old bills?

I recommend travelers checks when traveling internationally. If you want to carry cash, then I'd say keep it small, in case something happens. In the end, it doesn't matter what you carry. You can always convert it at an international bank there.

Adam's right, carrying traveler's cheques are best but, you should carry some new large bills USD 50 or 100's, the older and smaller bills were easier to counterfeit and the banks in China may not accept them.

The following is from the US Embassy in Beijing:

"The currency used in China is the Renminbi Yuan (RMB or ¥) and the value is pegged to the US dollar. The Yuan is divided into 10 jiao or 100 fen. Notes come in denominations of ¥100, 50, 20, 10, 5, 2, and 1. Make sure you exchange your left over Yuan before returning home because it can be exchanged only within China's borders. Travelers cheques, preferably in US Dollars, and foreign cash can be exchanged in cities at the Bank of China. The larger hotels and the special 'Friendship Stores' designed for foreigners will accept most western currencies for purchases. Major credit cards are accepted in the main cities at various establishments, but outside the major cities acceptance is limited. ATMs are scarce outside the main cities. "

Have a good trip.