The only way to know the performance difference for every situation would be to test it.
As for increasing the multiplier, all it does is increase the CPU speed by whatever the FSB is at (the original FSB not QDR or "rated" FSB).
Of course, you'll probably have to compensate for any increase in CPU or FSB speed with increased CPU Core and FSB voltages.
A higher FSB frequency will generally allow for better performance than a lower FSB frequency (all else being equal). But since it directly correlates to CPU speed, you just have to test it out and see what works best for you.