with an FX series processor you will get the best performance around DDR 1866 speeds with tight and stable timings and your HTT set to 2600.
in general FX processors gain more from tighter timings vs raw bandwidth (higher ram hertz) so if you cant hit much higher then 1600 your best bet is to just try and tighten timings as much as you can.
There is really very little to gain from overclocking ram other then bragging rights. It will not make games load faster, give you any noticeable FPS boost, reduce stutter est est.
In fact the only way to even see the difference is with benchmarks and otherwise you will just be consuming more power and shortening your hardware's lifetime.
Also know that no matter how fast your ram is, its limited by the HTT bus and as such you need to hit DDR 2166 or higher to saturate your HTT at 2600 mhz just fyi.
Most consumer DDR3 modules can tolerate 1.65v with decent airflow over the modules, but do check to see what your modules are rated for.
you will also want to go to overclock.net and look at their FX overclocking tutorials to understand the best ways to get OC'd ram stable with an FX processor (it can be a real pain).
the guy that posted above me doesn't know what hes talking about either. yes increasing your ram speed/tighter timings can lead to improved performance but even going from DDR 1600 to over DDR 4000 speeds will gain you single digit percents of improvement, which is some pretty abysmal scaling.
if your looking for more performance i would try overclocking your processor closer to 5ghz and getting a SSD. really once you go SSD you never go back.