CONDITIONING REGIMEN: Patients with matched, related stem cell donors receive cyclophosphamide intravenously (IV) on days -6 and -5 and undergo TBI twice daily (BID) on days -3 to -1. Patients with matched, unrelated stem cell donors receive carmustine IV over 3 hours on day -7, etoposide IV over 2 hours BID on days -6 to -3, and cytarabine IV over 3 hours BID on days -6 to -3, and melphalan IV over 30 minutes on day -2.