Fore Systems Inc. agreed to buy closely held Berkeley Networks for $250 million in stock and cash, acquiring equipment that speeds data on computer networks running on Microsoft Corp.'s Windows NT software. Pittsburgh-based Fore will issue 8.48 million shares for the Milpitas-based company. Including options and incentive payments, the agreement is worth as much as $250 million. Fore, whose equipment is used by large businesses and phone companies to combine voice and data on a network, gets two-thirds of its revenue from products based on asynchronous transfer mode, or ATM, technology. Berkeley's high-speed switches would broaden the company's product line. Fore shares rose 38 cents to close at $24.25 on Nasdaq.