Welfare Debit Cards Offer Benefits and Some Snags
New York City social workers are receiving hundreds of complaints from elderly, disabled and those who speak little English about new welfare debit cards that are replacing food stamps; Manhattan Borough Pres C Virginia Fields says welfare officials should suspend their plans to expand program until problems are worked out; Human Resources Administration acknowledges that some people are 'experiencing difficulties' with cards, in part because of computer problem that affected production; photos (M)