I'm assuming you have an Intel processor or a recent AMD processor with a locked multiplier. In that case, to overclock the FRONT SIDE BUS without overclocking the memory, you have to use a RAM divider. (emphasis on front side bus because you're actually overclocking the bus, which effects the speed of the CPU)
You can overclock the memory by using a different setting for the RAM divider.
3:2 means if the front side bus is 3, the RAM is 2... so with a FSB of 250 and a RAM divider of 3:2, the RAM will be running at 166 Mhz. With the same FSB and a RAM divider of 5:4 the RAM will be running at 200 Mhz. If you have the FSB at 200 and the RAM divider set to 4:5, you'll be running the RAM at 250.