I can see your point but it does look like a backwards step getting a dual core i3.
It may seem like that, but to put it in perspective an FX-8320 has 8 cores (technically four modules with two cores each but im trying no to techno babble you
) compared to the two cores of the i3, but the i3 has Hyperthreading which basically means each physical core can act as two virtual (logical) cores and work on two tasks.
WoW only utilizes three logical cores, which means an i3 is great in that respect as WoW will use three out of its four logical cores as opposed to three out of eight on the FX-8320 (in each case the is CPU power left over for other system functions).
The performance perspective is that each core on a current i3 is more powerful than a core on a FX-8320 (this is why AMD CPU's have more cores than Intel CPU's, they are slower per core so add more to try and even it out in software that can use many cores).
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However, having said all of that, I would actually recommend the FX-8320, as its the more future proof and having played WoW since beta I have seen it gain support for DX9, DX11, 64bit, Xfire, SLi and gain its ability to use three cores, so I can foresee it gaining the ability to use more in the future. And at the end of the day, when paired with a HD7850 the difference between an i3 and FX-8 will be hard to notice at 1080p.