6 cores and 4 cores provide a significant performance boost when Multitasking, that is working with multiple programs at the same time, ie having a spreadsheet, a word document and a dvd playing a movie in a window. There are a few programs that are able to take advantage of multiple cores such as rendering and photoediting.
Games are just starting to use more than 2 cores, should increase in the future, for the forseeable future 6 cores is probably overkill.
Added.
.. In most cases a few hundred MHz will not produce a noted increase in performance for vast majority of apps (are a few exceptions).
.. Intel CPUs have higher performance for a given clock speed, But AMD chips are cheaper an for "social" use are more cost effective.
For many that just use the computer for "social" use - web surfing, email, facebook and watching a movie a 2 core/4 threaded CPU is fine.
My laptop and both desktops are 4 core/4 threaded Intel CPUs and Have yet to hit a bottleneck (Not a gamer).