A meaningful comparison of computer hardware could only be done by common testing of an existing file with sufficient complexity to represent a 'calculation load'.
Long calculation times can occur any time toolpath parameters are set to very tight tolerances, or boundary constraints are too close to permit easy tool movement (when considering the finish allowance).
But any 4 core cpu should give you lots of oomph, with 6 gigs of ram....pretty much a standard 'higher end' PC nowadays.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)