NEW YORK (Reuters) - Wall Street's two main private regulatory arms said on Tuesday they have agreed to merge in a widely anticipated move aimed at reducing industry costs and streamlining oversight functions. Industry self-regulatory units run by the NASD and NYSE Group Inc. (NYX.N: Quote, Profile, Research), which operates the New York Stock Exchange, will combine self-regulatory operations into a single watchdog overseeing all securities firms doing business with the public in the United States. The new organization is expected to begin operations in the second quarter of 2007, and will consist of the current 2,400-person NASD organization and...