Overclocking the front side bus (FSB) automatically overclocks the memory since memory runs on this bus.
For instance, if you are running a system on 133MHz bus (266 for AMD) and you increase the FSB to 150MHz (300MHz for AMD), your memory is overclocked by 17MHz over default.
Some (most recent) boards also have the option to set the memory to + or - the host clock. This would be the case if you were running at 100MHz bus and had 133MHz memory. You would set the memory to HSTCLK+33 and it would be running at 133MHz.
Most DDR nowadays needs a little extra voltage boost to remain stable at higher frequencies as well.