Cyclophosphamide, Doxorubicin, Vincristine, and Prednisone With or Without Gemcitabine in Treating Patients With Previously Untreated Aggressive Non-Hodgkin's Lymphoma

RATIONALE: Drugs used in chemotherapy, such as cyclophosphamide, doxorubicin, vincristine, prednisone, and gemcitabine, work in different ways to stop cancer cells from dividing so they stop growing or die. Combining more than one chemotherapy drug may kill more cancer cells. PURPOSE: This randomized phase II trial is studying giving combination chemotherapy together with gemcitabine to see how well it works compared to giving combination chemotherapy alone in treating patients with previously untreated aggressive stage II, stage III, or stage IV non-Hodgkin\'s lymphoma.