The governing board of the University Medical Center Hospital Corporation is scheduled to meet officially for the first time Wednesday. Yet it remains to be seen just how strong a hand the panel actually will take in shaping the Charity Hospital successor for which it is legally responsible. Robert Yarborough, chairman of the newly constituted board, promises that he and his 10 colleagues will "dive in" and assert themselves in a project that began long before any UMC board appointees knew there would be a new, ostensibly independent corporation to oversee a new teaching hospital. "We've got to digest all the information that's out there and then decide how to proceed," he said.