This study will examine the safety and effectiveness of the drugs Gemcitabine and Herceptin, given in conjunction with radiation therapy, for treating patients with cancer of the pancreas. Gemcitabine is standard therapy for advanced pancreatic cancer. It damages tumor cells and may enhance the effects of radiation therapy. Herceptin is a genetically engineered antibody that has prevented some tumors from growing in patients with breast cancer. Because breast and pancreatic cancer cells share an characteristic related to how Herceptin works (similar HER-2/neu receptor proteins), it is thought that this drug may also inhibit growth of tumors of the pancreas.