Autodesk Maya - Exploring Physical Sun

Expanding on a tutorial l found online, and using Maya 2012, below is the first render after adding mental ray as the renderer and physical sun with the rays going through the roof opening on top.

Below is my basic setup from the Left hand side viewport

Then the render

Then with an area light added, scaled exactly to match the top opening, we render and get below

Enabling final gather and adding the settings below:

We get this render output

Enable jitter setting and increasing the max sample level from default 2 to 4 we get

And this is the render output (takes time)

CPU and memory load during render is shown below

Click here for link to original reference article