Flying over San Jose at 4,000 feet, Silicon Valley unfolding below, one sees the stretch marks of the digital boom. Acres and acres of new tract housing, neighborhoods in progress, push their way up the verdant hillsides of the rolling Santa Clara Valley, waiting to absorb the thousands of newly minted dot-com millionaires and almost-millionaires fueling the area's astonishing growth. Except for the size of the houses, the scene calls to mind the 17,000 homes dropped by the Levitt brothers onto the potato fields of Long Island to make room for young families beginning to prosper in that earlier, postwar era of remarkable expansion.