The goal of this clinical research study is to learn if giving busulfan and fludarabine before a stem cell transplant can help control the disease better than the standard method in patients with leukemia, lymphoma, multiple myeloma, MDS, or MPD. In this study, 2 doses of busulfan will be given 2 weeks before a stem cell transplant followed by 4 doses of busulfan and fludarabine during the week before the stem cell transplant, rather than the standard method of giving 4 doses of busulfan and fludarabine only during the week before the stem cell transplant.