The specter of nursing strikes is looming on both coasts, as newly empowered nurses' unions confront hospitals pressed to cut costs amid changes in health care financing. In New York, over 6,000 registered nurses are poised to walk out of three of the city's most prestigious hospitals before the year's end, mainly over changes to their health benefits and what they say are strains in staffing. The hospitals — Mount Sinai, Montefiore Medical Center and St. Luke's-Roosevelt Hospital Center — already are contracting for strike replacements at more than double normal wages.