God Rays?  There is no better God rays than the one's invented by Frozencry all those years ago at Bryce5.com. Here is a link to the original formula for God rays in Bryce:

http://www.bryce5.com/details.php?image_id=1144

I used this in my most recent render link below:
http://www.renderosity.com/mod/gallery/index.php?image_id=1962886

Hope this helps.