THREE years after the Olympic medals were handed out in Seoul and the fans and athletes from 161 nations departed, the city is still catching its breath.Visible changes were immense. To host the most expensive Olympics ever, at a cost of $3.3 billion, a 25-mile stretch of the Han River was transformed into recreation areas, a permanent 420-acre Olympic Park along with 3,700 apartments was built, and the airport and highways were expanded. A whole new urban subcenter was created. The Korean construction industry also adopted an international style, improving the architecture of a once drab city with new techniques, better materials, and fancy exteriors. Seoul was the second Asian city to host the Olympics, and as Tokyo did in 1964, it used them as an economic and political springboard. The international image of South Korea was enhanced - its capital markets and foreign currency were liberated, its tourism boosted - and the games served to agitate moves toward democracy, says Kim Jong Gie, director of research planning and coordination at the Korea Development Institute, which conducted a study on the effects of the Olympics. In addition, South Korea achieved a diplomatic success by outmaneuvering North Korea. The games allowed China and the Soviet Union to improve ties to Seoul. But city residents also changed, becoming more international. Public spitting occurs less frequently and driving habits are slightly improved, says Dr. Kim. "Before, the hot-tempered Koreans would not stand in line and would rush for the door when a bus pulled up. Now they are more polite," he adds. The games gave Koreans a new self-identity, says city planner Kang Hong Bin, with a renewed interest in their own history and folk culture. Negative effects linger, however. The development enhanced south Seoul, causing resentment from the oft-neglected north side. And residents of slums known as "moon villages" were forceably moved to the city outskirts, where they still live in vinyl greenhouses.