Four-hundred mg is generally as high as most doctors will go (as per the PDR, I believe), but I've known people who were prescribed on as much as 450 mg.
Going higher than this recommended limit increases risk of seizures.
Your doctor also needs to assess your general medical condition, any other meds that you may be on, etc.
My personal experience is there's no difference between generic and brand-name, aside from cost (and co-pay).
The highest dosage of Wellbutrin SR is 400 mg per day. I agree with LCC's comments.
I really suggest with your pdoc or your GP before thinking about raising meds. Reactions are indivualistic, so speak with your doctor, that's the only i can suggest.