 RATIONALE: Giving low doses of chemotherapy before a donor stem cell transplant helps stop the growth of cancer cells. It may also stop the patients immune system from rejecting the donors stem cells. The donated stem cells may replace the patients immune cells and help destroy any remaining cancer cells graft-versus-tumor effect. Sometimes the transplanted cells from a donor can also make an immune response against the bodys normal cells. Giving a monoclonal antibody, such as alemtuzumab, before transplant and tacrolimus and methotrexate after transplant may stop this from happening. 