I would definitely suggest that you go along with your doctor on this one. For one thing, if it is a curable form of NHL, then it is probably also something that can progress quickly.
Someone else will have to tell you about Treanda, but Rituxan should be comparatively mild. Most people experience their worst reaction, hives and feeling like they have the flu for a day or so, the first time they take it. After that, the reactions are milder. Some people have a greater reaction so be sure to tell your nurse and/or oncologist about your reactions. And, talk to your doctor about pre-treatment for side effects with steroids and benedryl.