FAQ? Mac distributed/multiple processor solutions?

I felt this must be an FAQ but I don't see it anywhere: my apologies  
if I've missed it.

I have what I believe is known as an "embarrassingly parallel"  
problem comprising a large number of repetitions of a single  
(lengthy) calculation that generates a boolean result and I am simply  
interested in the final proportion of true to false runs. This  
obviously lends itself to parallel computation and I'd appreciate Mac- 
specific pointers to both simple distributed and multiple-processor  
options here. I am working on an iMac G5 and could access (at home -  
i.e. not on a LAN) another G5 and a G4. I've come across the R/MPI  
package but would appreciate advice as to how easy this is to set up  
(would it actually be simpler to divide the job "manually"?).  
Alternatively I have an option to acquire a MacPro for this work and  
would appreciate guidance as to whether it's possible to leverage  
multiple processors?  I'm aware R itself is not currently  
multithreaded (whilst having only a lay understanding of what that  
means).