FX is a much more powerful series overall with a higher multi-threaded performance. APU's will always have less CPU power.
Quick browsing:
http://cpuboss.com/cpus/AMD-FX-4300-vs-AMD-A8-6600K
About the same specs except that FX has L3 cache while APU doesn't.
I can't see much higher FX performance here. FX will be faster but L3 cache is not helping in everything especially that AMD uses large but really slow cache.
Here is also 4.2GHz FX vs 3.5GHz i3:
http://cpuboss.com/cpus/Intel-Core-i3-4330-vs-AMD-FX-4350
Results are similar in most tests ( also not included in link ). When I was comparing APU to i3 then I had to set ~4.3-4.4GHz to get about the same results as 3.5GHz i3 ( depends from test ).
Also sorry for off topic.