This is a phase I study using Intensity Modulated Total Marrow Irradiation (IM-TMI) in addition to a chemotherapy regimen in preparation for an allogeneic stem cell transplant for advanced hematologic malignancies such as acute myeloid or lymphoblastic leukemia, high grade non Hodgkin's or Hodgkin's lymphoma, chronic myelogenous leukemia, and plasma cell leukemia. Because the subjects participating in this study have a disease that is severe and has a high risk of relapse even after transplant, the investigators propose to use a chemotherapy regimen (fludarabine/busulfan), the name for the combination of chemotherapy drugs that is given to patients prior to transplantation of the donor stem cells, along with intensity modulated radiation (IM-TMI) to the bone marrow. Total body irradiation (TBI) in conjunction with chemotherapy is a standard of care as a pre-conditioning regimen prior to bone marrow transplant (BMT) in patients with hematologic malignancies. However, TBI can cause severe side effects due to irradiation of organs such as the lenses of the eye, whole brain, lungs, liver, kidneys, heart, small bowel and oral cavity. IM-TMI allows for the delivery of adequate doses of radiation to the bone marrow while sparing other organs and therefore limiting radiation side effects. The irradiation, along with receiving the chemotherapy drugs will suppress the subject's immune system and kill off tumor cells, but will also intensify the effect of the conditioning regimen thus allowing the bone marrow transplantation to have a greater chance of being successful.