Well...Intel clock-for-clock, and $-for-$ will actually out render AMD (check-out Tom's Hardware site).  Also, the i7 Intel processor family is actually a 4-core processor with 4 advanced Hyperthreaded  virtual cores. I know that 64-bit Vista Home Premium will see 4-cores max, so do some research concerning what OS to use. Vista Business 64-bit (or 32-bit) will see 8 cores.

The new Intel i7 MBs and processors are a bit overpriced -- as far as I'm concerned.

The newer AMD processor/MBs do have a better overall bang-for-the-buck vs. Intel, if rendering is not your primary focus.

I'm running a Mac Pro with 2 x 2.8 GHz Xeon quad-cores and 14 GB of RAM.  I use Vue7I more than Carrara now days, so I do most rendering work on the PC (Vista Business 64-bit) side of the Mac. Carrara does render very fast under both Windows or Mac OS (which I prefer).  I just wish more applications were 64-bit on the Mac (Vue is still 32 bit on the Mac). You might want to consider the Mac Pro 8-core, as it is a hardware bargin -- compared to other 8-core Xeon PCs.