Most mobos have to OC the RAM to get above 1600. 1866 RAM is actually DDR3 15000. The OC is a natural product of the DDR3 architecture (DDR stands for Double Data Rate).
The SB CPU is optimized to use the RAM via XMP. It sets the fastest stable clock rate automatically. The XMP settings on my mobo default to 1600 speed and I had to reset my priofile in the BIOS to get the 2133 speed that the RAM is capable of.
OC ing the RAM is not the same as OC ing the CPU, however, if you are buying into the 2500K platform why would you do so without OC ing it? You'd be better off with the plain i5 2400 or 2500 chips on an H67 mobo if you don't plan to OC.
The ASUS Max IV is a great board but, why spend that much if you don't want to OC?