Don't worry about asking dumb questions, that's why we're here
First, here's a little background on front side bus. For an Intel CPU, the FSB value is quad-pumped, or multiplied by 4. For example, an Intel Pentium 4 2.8 GHz w/ 800 FSB would have the motherboard setting set at 200 MHz * 4 = 800 MHz. AMD CPU's are a bit different, and are multiplied by 2 instead of by 4. For your particular CPU, the FSB value is 333 MHz. Therefore, to answer your question you motherboard value should be set at 166 MHz, so that 166 MHz * 2 = 333 MHz. Also, when finding overall operating frequency, you take the CPU frequency, mulitply it by the multiplier for your particular CPU, and then you arrive at the operating frequency value. For your XP 2600+ it would look like this.
166 MHz * 11.5 = 1909 MHz, or 1.91 GHz.
Does that help a bit?