I run C6Pro on a Dual Core Athlon64x2 processor with 2GB Corsair RAM.  I find just the opposite.  It took almost twice as long to render the same scene when all I had was a single core Athlon 64 of about the same speed.  I run Windows XP, so I am not sure if it might be something OS related.  I do know a friend of mine with a system very similar to what you described said that my system seems to run C6 Pro faster than his Mac version.  A lot of folks don't realize this but there is a difference between AMD and Intel double and quad core CPUs.. AMD multi cores have seperate cache for for each core.  The Intel chips share cache between 2 cores which slows down the chip.  This might be the issue since I believe the G5 uses an Intel chip.

Quote - Howdy --

New Mac, New Version of Carrara, New Problems!

So, here's the deal.

If I render a scene in Carrara using multi-core rendering (with "Enable Multi Threading" ticked in the Rendering tab in the Rendering Room), the scene will render significantly slower than it renders with a single core ("Enable Multi Threaing" not ticked in the Rendering tab). 

I say again:  using multi-core rendering is rendering significantly slower than using single core rendering.

AND this is happening in Carrara 5.1 Pro on a G5 quadcore using Tiger, and in Carrara 6.03 Pro, on a BRAND SPANKING NEW MacPro Octo-Core using Leopard.

On the Daz forums I see that Kix has posted a benchmark test and it seems that other folks are having this same issue.

Why would this be?  Is there any way I can get Carrara to actually take advantage of this screaming new machine?  (Or for that matter, the screaming old quadcore?)  Is it just a Mac thing?  Did Eovia (and now Daz) release a substandard build on the Mac?  Or is it something that appears on PCs as well?

Anyone have any ideas? 

Thanks!
Dex