AMD's 8 core is 4 physical cores with 4 "logic" cores, intel calls this hyperthreading so a quad core i5 = an 8 core AMD as far as number of cores.
The i5 4670 beats the FX-8350 in pretty much every way except the price. Now it is not twice as fast as some may imply but it is a noticable bit faster.
As far as the motherboard goes you just get one that has the features you are looking for, a 990FX chip would be the best equivilant to a Z87.
For my needs i bought an fx-8320 because it had enough power for my needs vs the price point I was wanting to spend. If I had a bigger budget and was not aleady using an AMD board I would have spend the extra money for an i5.
Unless you look at the facts.
I used to be an AMD fanboy also.
Take an Ivy bridge i5, over clock it to 4gig. (only 4 cores)
Compare it to an 8 core AMD, running @ 4Ggig.
They bench very close.
I haven't even played the i7 card yet.
The reason they clock near the same is not because a 4 core i5 is twice fast per core as an 8 core AMD, it is because they actually have the same number of cores. AMD 8 core = 4 physical cores, each physical core = 2 logical cores. The i5 uses hyperthreading and thus each physical core = 2 logical cores. The difference is that intel advertises it is a quad core and AMD advertises it as a octa-core.
The amd logical cores is similar but not the same as hyper threading, the end result is being able to process the same number of threads at one time (an 8 core AMD vs a 4 core Intel with hyper threading). I was incorrect in that the quad core i5s do not have hyper threading, the dual core i5 does but not the quad core.