FSB Spread Spectrum
This BIOS feature allows you to reduce the EMI of the front side bus (known alternately as the FSB or processor bus) by modulating the signals it generates so that the spikes are reduced to flatter curves. It achieves this by varying the frequency slightly so that the signal does not use any particular frequency for more than a moment.
The BIOS usually offers two levels of modulation - 0.5% or 1.0%. The greater the modulation, the greater the reduction of EMI. Therefore, if you need to significantly reduce the front side bus' EMI, a modulation of 1.0% is recommended.
In most conditions, frequency modulation via this feature should not cause any problems. However, system stability may be compromised if you are overclocking the front side bus. Of course, this depends on the amount of modulation, the extent of overclocking and other factors like temperature, etc. As such, the problem may not readily manifest itself immediately.
Therefore, it is recommended that you disable this feature if you are overclocking the front side bus. The risk of crashing your system is not worth the reduction in EMI. Of course, if EMI reduction is important to you, enable this feature by all means. But you should reduce the clock speed a little to provide a margin of safety.
If you are not overclocking, the decision to enable or disable this feature is really up to you. But unless you have EMI problems or sensitive data that must be safeguarded from electronic eavesdropping, it is best to disable this feature to remove the possibility of stability issues.
