graininess in part of rendered image

I've been playing around with irender nxt for the past 2 months. Yesterday i rendered a model using path tracer engine with indirect lighting method:gather. After running it for 400 passes I still see some graininess in part of the rendered image. My questions are:

1. Is it because of the indirect lighting method? or the path tracer engine?

2. do i need to run more passes?

3. how important is it to use indirect lighting method when rendering? would running just the path tracer engine and no indirect lighting specified be enough?

Did I overdo it? or miss any steps? Any input is greatly appreciated

thanks

sandy