Most motherboards have a set number of ratios for memory 1333 | 1600 | 1866 etc.
If you have not changed the busclock from 200MHz, these ratios will remain the same.
If you overclock the bus above 200MHz, for example 250MHz, the memory ratio will increase by 250 /200 = 1.25 so you get 1667 | 2000 | 2333 etc.
You may need to reduce the selected ratio from 1600 to 1333 if you were using 1600 ram and a busclock of 250Mhz in this example.
Also the HT and NB ratio will increase and may need reducing. Ensure you fix the PCIe clock at 100MHz when overclocking the Bus clock.
This is a simplistic case but shows the effect of increasing the bus clock.
I have successfully overclocked the bus to 302MHz in one of my saved bios profiles but use 218MHz as a 24/7 overclock. My overclock uses patriot 2133MHz ram at 2030Mhz and a bus clock of 218MHz at 21.5 multiplier.