Depending on your combination of processor and motherboard, you may also be able to change the actual internal frequency multiplier of the CPU itself, which multiplies the FSB speed to arrive at the actual speed of the CPU in GHz or MHz.
For example: an Athlon XP 3000+ 'Barton' processor has a multiplier of 13 and uses a FSB speed of 166Mhz. 166Mhz x 13 equals approximately 2.16Ghz. Change the multiplier to 13.5 and you get (166MHz x 13.5 =) 2.24 Ghz.
Although a small change to the multiplier has a larger proportional effect on your systems speed than increasing the front side bus a considerable amount, the actual performance advantage of increasing the CPU multiplier is not so simple. As the multiplier purely effects the processor's performance, the performance gained by increasing it is not felt system-wide, as is the case with overclocking the FSB. It merely enables the processor to do more work per second. In fact, it may well serve you better to decrease the CPU multiplier in order to overclock the FSB to a higher frequency than would otherwise be possible.
This is something to consider if you have high-quality memory that is rated for greater speeds than the FSB of your computer requires. Many memory producers make DDR memory that is capable of running at much higher frequencies than modern computers normally use, specifically for overclocking purposes.
The option for changing the multiplier is found in the BIOS in the same location as the FSB options, generally the 'frequency\voltage control' section.
Raise the multiplier only a step at first, in concert with overclocking the FSB. Find the maximum stable speed you can achieve, then benchmark. If you have high-spec memory, consider lowering the multiplier and increasing the FSB, then compare the new set of benchmarks to the previous ones.