Ok so I'm thinking that your question is asking if the FSB is restricted more by the mobo or is it possible that just switching cpus could increase FSB.
I'm gonna try and answer that, (hopefully its more what you were asking):
Basically the answer is again that it depends on the cpu, your cooling system, and the cpu multiplier.
Basically until you unlock a P4 the multiplier is fixed at ____ (23 in my case) meaning that the only way to increase the FSB is to increase the cpu clock, which raises cpu speed along with the FSB and also increases cpu temp! so this is where you need some extreme cooling, I mean yeah you could crank your p4 to 4Ghz+ and watch it die within seconds achieving a huge FSB but that doesn't make any sense. If you unlock the processor, you can change the multiplier which in turn should allow you lower your cpu speed, and make it possible to increase FSB with a lesser increase in cpu speed, which in turn lowers temperatures. I've been unsuccessful in unlocking my p4 cause I don't want to pull pins or paint them. (if anyone has suggestions on other ways to unlock a p4 3.06 northwood let me know cause I have yet to read about another way to unlock it).
Ok as for AMD pretty much all either come unlocked or can easily be unlocked, I think AMD designs there CPUs more for the OCer in that aspect... but yeah because you can change the multiplier more ore less right off the bat in an AMD, its maybe easier to get the FSB initially higher on an AMD, but who knows in the end if you have both cpus unlocked with identical cooling systems.
So unfortunately I don't think your question has an obvious and direct answer that is applicable 100% of the time.
If I understand what you're asking, you will get varying OC results depending on the chip you have (not considering any other factors). Chips from the same exact batch can vary greatly in what they can run at.
cause in the end it really depends on IF you get a "better" cpu from the batch and furthermore on the rest of your system setup.