You'll have to up the bclk to 133 from 100 with the 1866 mem divider selected.
If that MSI fm2 board is anything like the 3 different fm1 boards i've tried you could run into other problems as well, such as vga no longer working and losing the use of the first 4 sata ports.
Although, to your question, I would still personally suggest getting 2133 mem.
There are many good memory choices now (mushkin, gskill, kingston hyperx, samsung'1600 stuff overclocks well', etc.).
I'd suggest getting everything stable at 100bclk 1866 mem before even attempting 2133 however.
EDIT: Apparently some of the stuff
I mentioned above has been fixed since fm1. upto 2400 mem speed with 100 bclk is possible according to that link.
A8-3870 @3.3GHz 1.3125v
16GB 1866 9-9-9-23 1.35v
Linux software/gaming exclusively
Afraid to buy radeons because they might be voltage locked.
I want to lower/raise voltages.