The goal of this clinical research study is to learn if there is a difference in transplant outcomes between two different doses of melphalan given in combination with fludarabine followed by transfusion of a related or unrelated volunteer donor's peripheral blood or bone marrow progenitor cells (allogeneic stem cell transplant) in patients with multiple myeloma. This study will also look at whether treatment with a antibody called rituximab against a specific type of lymphocyte (B cell) will reduce the risks of developing graft versus host disease after transplant. The safety of these treatments will also be compared.