The Ford Motor Company, by agreeing yesterday to buy the car operations of Volvo A.B. of Sweden for $6.45 billion, has taken its biggest step yet toward becoming a large-scale seller of high-profit luxury cars as well as more humble vehicles like the Escort and pickup trucks. Ford has already bought Jaguar and Aston Martin in recent years, but the acquisition of Volvo, a favorite of affluent urban families in the Northeastern United States and northern Europe, represents a far greater bet on the luxury market. Sales of luxury cars are rising in the United States and Europe while auto makers' profits on less expensive cars have evaporated as low-cost manufacturers in places like South Korea flood markets in the United States and elsewhere with ever- cheaper cars.