If you have a brokerage account at either Paine Webber or Kidder Peabody, you may want to get ready for some changes. Paine Webber announced it will buy Kidder Peabody from General Electric for $670 million. The move will make Paine Webber the fourth largest brokerage firm in America, after Merrill Lynch, Smith Barney and Dean Witter. On the bright side, the move should strengthen the firm's already considerable research and investment resources. On the down side, though, some analysts speculate that as many as half of Kidder's 5,000 employees may be laid off eventually.