PURPOSE: This randomized phase III trial is studying salvage chemotherapy using dexamethasone, cisplatin, and gemcitabine to see how well it works compared to dexamethasone, cisplatin, and cytarabine given before autologous stem cell transplantation in treating patients with relapsed or refractory aggressive non-Hodgkin's lymphoma. This trial also is studying giving rituximab as maintenance therapy to see how well it works compared to no further therapy after stem cell transplantation. Rituximab was added to both salvage treatment arms for CD20+ patients in a protocol amendment in 2005.