Arctic Silver really is the best thermal paste out there.  I upgraded a Power Mac from four cores (2 CPU with 2 cores each) to eight cores (2 CPU with 4 cores each).  With the Arctic Silver the machine is actually running cooler than stock!  Be sure to match this with the Arctic Silver Articlean kit if you are doing an upgrade rather than a clean install; it makes cleaning the old thermal paste off of the old heat sink/CPU a breeze.