for the most performance per dollar spent the amd fx 6300 and athlon ii x4 760k are sitting right in that sweet spot... and have been for a while. that said the is a finite limit to the power you'll get out of an amd chip (they're somewhere around Nehelam level performance, maybe a little bit slower)
The result is if you want or NEED more power then an AMD can give you you'll spend more money for smaller gains. A haswell i5-4670k comes in at $230, while the Athlon II x4 760k comes in at $73 and the FX 6300 comes in at $110. Those AMD cpus are 31% and 47% the cost of the haswell. In single core performance they're around 65% the speed of a haswell. the 6 core fx in fully threaded tasks actually closes the performance gap pretty quickly coming in within 10% of that haswell thanks to the 2 core advantage... of course few things are fully threaded so that's a questionable advantage.
That said if you want pure horsepower under your hood, you're not going AMD. Intel has a sizable advantage in single core performance. and most programs are single threaded.
if you're sitting on an old nehelam core i cpu, you'll see an advantage going to an i5-3570k or i5-4670k. you won't see much of an improvement going to an fx processor since they're roughly the same speed. if you're on an old core2 cpu you'll probably see a performance increase going with either. if you're on a phenomII quad or 6 core its doubtful you'll see enough of an improvement to justify going to a fx cpu...