My trusty crunching rig died the other day. I guess years of abuse and over clocking and finally one of the VRMs let the smoke out.

My question is which would be better for world community grid, a six core x58 or a 8 core FX.

PC also doubles as a media center pc.

WCG badges are earned by the hour of CPU processor time, so the more threads the better.
The 12 threads of the x56xx CPU should out score the AMD also.
 
If thinking of buying a new board, CPU look at x79- LGA 2011. Those CPUs SandyBridge or IvyBridge have AVX.
There are used server hexcore xeons that are around 90$ that overclock very well. E5-1650 and the 1650V2 (130$)... Sandy - Ivybridge.
That you can reuse your DDR3 memory.
 
Newer setups have AVX, AVX2, F16c, FMA3  These extensions really help out when doing BOINC projects.
Now if looking for something newer the Z370 boards with an I5-8600k or i7-8700k hexcore 240$-339$ are blazing fast doing BOINC
But you will need to buy new DDR4 memory.
 
Another way is the x99 - 2011V3 socket. The hexcore-K i7-5930k CPUs are down to around 200$ and the boards used go 100-200$ depending.
They too will need DDR4
 
AMDs implementation of AVX, AVX2 is poor compared to Intels 
 
 
 
 

Awesome reply Bill..