What is the difference of DDR3 1600 and DDR3 1866



It depends on the latency. If the latency scales with the frequency, then the modules will perform very very close to the same. That linear movement looks like.... DDR3-1600 CL9 performs the same as DDR3-1866 CL10 performs the same as DDR3-2133 CL11 performs the same as DDR3-2400 CL12. If you are comparing DDR3-1600 CL9 with DDR3-1866 CL10 then there is no real difference. You will need 1866 modules with latency the same, or lower, than the 1600 modules you want to compare to in order to say it is "faster".

Keep voltage in mind also. At 1600 or 1866 frequencies you don't see many that are above 1.5v. There is absolutely no need for the 1.6v or 1.65v modules (outside of serious tweaking) that typically come with 2133 or 2400 modules.