RATIONALE: Drugs used in chemotherapy, such as temozolomide, busulfan, and O6-benzylguanine, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving more than one drug (combination chemotherapy) may kill more tumor cells. O6-benzylguanine may also help temozolomide work better by making tumor cells more sensitive to the drug. Radiation therapy uses high-energy x-rays to kill tumor cells. Giving chemotherapy with a peripheral stem cell transplant or bone marrow transplant, using stem cells from the patient that are genetically-modified in the laboratory to protect them from the side effects of chemotherapy, may allow more chemotherapy to be given so that more tumor cells are killed. Giving combination chemotherapy and radiation therapy together with a peripheral stem cell transplant or bone marrow transplant may kill more tumor cells.