what else is running on your puter during rendering?

Have you tried increasing the priority of Bryce task a little?

If you have an anti-virus that is bloatware and uses CPU cycles constantly while eating your RAM, consider switching to NOD32. (if you're on a PC)

and then of course see if you can get even more RAM stuffed into that puter.

If all fails, try Carrara. Much faster renderer. There's entry level versions like Carrara 3D Basics or 3D Express for small budgets. And Purplus has them too. http://www.thebest3d.com/carrara/resources.html
  can give you some ideas also about what's possible. Carrara has gone a long way with stuff like Mountains, plants, skies...

Oh, and this kind of volumetric shadows, easy. EVen a 2D post tool like Dogwaffle could help there.

What's the dimensions in pixels of your render?

-Philip