a rather broad question.
GHz is still king. however you cant compare one architecture to another.
EG: a 3.4GHz 4 core Ivy is much slower than a 3.4GHz Sandy E 6 core.
more to do with architechture than the added 2 cores
on the other hand a dual Xeon will get its butt kicked by a single 6 core until you get way up there is GHz with the Xeons. (basically 2x 3.1GHz 8 core)
right now the best purchase is the 6 core Sandy E
preferably the 3930k overclocked to 4.5GHz.