There are a couple of snags which your test doesn't take into account.

First off speed can vary quite a bit between different brand and model motherboards.  Asan example, both my wife and I have the same CPU a Phenom X4 9600.  I use a higher end MSI K9A4 Platinum motherboard, wheres she uses a lower cost board from Asus.  I ran the test on both machines and mine got a speed of 4:09.  Hert machine got a 7:24.  Both machines have 4GB of RAM which brings me to the next item.  Memory will also affect speeds.  Not just the amount, but the type and memory speed.  For instance, my wifes system uses DDR memory, but mine uses DDR2 Dual Channel RAM.  Major speed difference.  There are other variables as well.  One is the Hard Drive speed.  Carrara will always end up going to the hard drive no matter what you do.  A slower drive will slow things down considerably, whereas a faster drive helps a lot.

Just a few things to keep in mind when folks do these tests.

Quote - The idea is to have a point to compare machine to machine, so no change can be done before the render. The settings are for a good result with GI without lots of splat shadows artifacts.
If everyone use your system to render the scene file we can look at it and see how other system performe the same scene... so we can determine the real diference between a Core 2 Duo 2.4 and a Core 2 Duo 3.0 for example.