When it all comes down to the basics, every processor on the market has a true FSB and a multiplier. The true FSB is what you need to look at, not the FSB after the multiplier is added. Here's a breakdown of true/market FSB for Intel and AMD:
<UL>Intel Pentium 4 Processors
<LI>4x100 MHz true FSB = 400 MHz FSB
<LI>4x133 MHz true FSB = 533 MHz FSB
<LI>4x200 MHz true FSB = 800 MHz FSB
</UL>
See? The multiplier on Intel Pentium 4's is 4. The true FSB is multiplied by four, theoretically raising the market FSB to numbers like 800 MHz.
<UL>AMD Athlon XP Processors
<LI>2x133 MHz true FSB = 266 MHz FSB
<LI>2x166 MHz true FSB = 333 MHz FSB
<LI>2x200 MHz true FSB = 400 MHz FSB
</UL>
As you can see, AMD Athlon XP processors have a multiplier of 2 on the processor. The true FSB of the Athlon XPs are the same as Intel Pentium 4s...Intel simply jumped from 133 MHz to 200 MHz.
<UL>AMD Athlon 64 Processors
<LI>???x200 MHz true FSB = 1.6 GHz
</UL>
The Athlon 64 processors have an odd FSB. There is no multiplier, but theoretically it should be 8. 8x200 = 1600.
Every processor on the market shares a true FSB with a higher clock speed processor. A 333 MHz FSB Athlon XP has a higher FSB than a 533 MHz FSB Pentium 4. The Athlon XP's true FSB is 166 while the Pentium 4's is 133. It's all in the multiplier...the FSB listed on processors is basically for marketing. The true FSB is all buyers need to be aware of.
The FSB supported on the motherboard is a good fact to know, but knowing which socket the motherboard is is a bigger deal. The 1.6 GHz FSB motherboard will only take Athlon 64's and the 800 MHz FSB motherboard will only take Pentium 4's. Of course, both share the same true FSB.
I hope this helps.
-Lasereth