I would go with 1600 ram personally in 2 x 4 config. I would also buy an aftermarket cooler such as Hyper 212 or Xigmatek GAIA as I'm guessing you will want to o/c.
You didn't mention what case, but with 2 gpus, be sure you get a full tower with good fan options. You will be putting out a lot of heat, and I would suggest at least 3 x 120/140mm intake and 2 x 120/140 exhaust, plus good cable management. You might also want to look at cases that provide fan mounts on the side panel (200mm maybe?) so you can blow directly over the gpus. If you use a side panel 200mm, reduce the intake to 2 x 120, but leave the exhaust alone.
FX8350 will hit 1866 , but you will see no difference in performance.
Stick with 1600 mhz.
1600 vs 1866 are the same price category, most amd processors DO benefit from it (though I admit it will not always be very noticable) AND taking the higher speed will futureproof you a bit more if you would want to reuse the sticks in another system later.
If the price point is there it's fine, but there is no world difference between 1600mhz. and 2133 mhz.
Agree, you won't get 5fps more or notice faster handling today, but you will be happy if you build a new system (somewhere before DDR4 is reasonable) and won't have to spend anything on faster ram. But this is not a discussion thread, so let us quit the nitpicking and may the OP decide