Actually the new Mac Pro has 8-cores (2X4 core Xeon “Nehalem” processors) which, along with 8 virtual Hyperthreaded cores results in 16 "cores." The i7 is 1X4-cores plus 4 Hyperthreaded cores, for 8 "cores."
 Let's not compare Apples to oranges though. The Xeon “Nehalem” processor is not, I repeat, NOT the same as the i7.  The i7 is pretty comparable in doing rendering -- clock for clock, but the mac is a workstation level PC, and one could use the Xeon “Nehalem” processor based Mac as a powerful server. The Mac Pro is first and foremost a powerful graphics workstation. BTW, 3D applications like Vue are graphics applications -- just not 2D graphics applications.
My Mac Pro is not the Xeon “Nehalem” processor base Mac, but the Jan 2008 model. Doesn't matter to me, the 8-cores still render like about 16-cores worth of RenderCow activity (RenderCow is Not very efficient!).
The only significant difference between a brown box PC and my Mac Pro is the OS. Except, my Mac is also the most stable Windows platform I've ever used.