http://ca.msi.com/support/mb/760GMP21_FX.html#support-cpu <- compatibility (zambezi = bulldozer, vishera = piledriver)
a CPU upgrade is definitely a good idea but I can't think of any reason you would need an 8-core CPU. more physical cores are definitely beneficial for many server, HPC and niche workstation applications (eg. video encoding) because those applications are developed for concurrency (to run in parallel). most of the applications/games we use are single-process so even though they
use multithreading, they are not developed specifically for concurrency so a lot of the code is old-school serial and what matters for serial processing is clock speed.
Having said that, there is still a benefit to buying a 6 or 8 core CPU, and the benefit is that you are able to stress-test to isolate the performance of each individual core, shut down the laggards and run just the four fastest. It's basically like binning your own CPU. You could of course leave the cores running, but because of the stuff mentioned in the previous paragraph there's really no point because more active cores = more heat added and that means less clock speed, which you will want more of rather than less.
So, taking all that into consideration, the best value would be buying the most expensive FX you can afford, restricting it to the four fastest cores and bumping up the clock multiplier until you max out the cooler's potential.