The i3 is the fastest on 2 cores. @54watts
The fx4300 @ 95 watts is about the same speed when all 4 cores are used, and half the speed on 2 cores.
The fx 8320 @ 125 watts is twice as fast as the other 2 but it takes all 8 cores running at the same time to do it.
So in 2 core operations the i3 is twice as fast.
The only reason, and it is slim, to go with an 8 core AMD, is for video editing, or other multi core intensive programs
No game uses 8 cores to my knowledge
AFAIK most games still use 1 or 2 cores, but that is changing slowly.
I flight Sim, and it only uses 2 cores, so hands down I would go with the I3 with those choices.
I choose to go with an I5 though