The high doses of chemotherapy and radiation therapy given prior to a standard bone marrow stem cell transplant can make a patient very sick and also increase the chance of getting severe GVHD. In this research study we are going to rely mainly on the donor cells to kill the cancer cells, and patients will receive dosages of chemotherapy that are lower than the usual dosages. It is thought that this low intensity conditioning will make the transplant safer without risking more relapses, however it is not know whether low intensity conditioning is safer than standard conditioning.