Randomized phase III trial to compare the effectiveness of two regimens of combination chemotherapy followed by rituximab or observation in treating patients who have stage III or stage IV low-grade non-Hodgkin's lymphoma. Drugs used in chemotherapy use different ways to stop cancer cells from dividing so they stop growing or die. Combining more than one drug may kill more cancer cells. Monoclonal antibodies such as rituximab can locate cancer cells and either kill them or deliver cancer-killing substances to them without harming normal cells. It is not yet known which regimen of combination chemotherapy, with or without rituximab, is more effective for non-Hodgkin's lymphoma