The latest American Express ad campaign starring longtime spokesman Jerry Seinfeld began not on television but on the company's own Web site. The two, four-minute "Webisodes" co-starring an animated Superman have pulled more than 2 million visitors in through AmEx' Internet portal. "The changes we are making are in line with the changes consumers are making, like spending time in front of the computer instead of the TV set," says John Hayes, the chief marketing officer of American Express. A few weeks ago, AmEx started running the first Web site in conjunction with TBS reruns of Sex and the City, a series beloved by the same young urban professional crowdthat made Seinfeld such a hit.