BEIRUT -- King Faisal of Saudi Arabia, a moderate Arab leader who became one of the world's richest and most powerful monarchs, was assassinated today by his mentally unsound nephew, the nation's official Riyadh Radio announced. Death of the 70-year-old Faisal could have drastic effects on the Arab-Israeli conflict, with unforeseen changes of future oil prices that Faisal helped raise by 400 per cent. It came at a time when United States influence in the Middle East had just suffered a major