Your worst case scenario, rendering in passes and composing everything in a 2D program.  Let's analyze your scene.  Are your objects displacement/bump map intensive and are they close or distant in relation to your scene?  If close, then these maps have some validity to be present.  If distant, do away with those maps entirely, or use normal mapping, they less resource intensive.

Are there many characters with transmap hair?  Transmaps are notorious for slowing down renders, especially where IDL is concerned.  If you are not doing a closeup, or portrait, do not make the transmap hair visible in raytracing. 

Raytrace bounces, do you need that value at 4?  Are there reflective surfaces?  If not, you can reduce raytrace bounces to 2. 

Number of pixels, are they necessary at 6?  Could you trim that back to 4? 

What is your reason for using 300 dpi?  Do you intend to print your images?

BTW, I do agree with the others concerning that huge bucket size.  Calculate 5000 x 5000, and that is the number of pixels you are analyzing per bucket.