the FSB limit is generally imposed by the motherboard, not the CPU - if you set the CPU multiplier to x1 (that's probably not possible, but it's an illustration

) then as far as the CPU would be concerned it would only be running at 450MHz. CPUs tend to be constrained by a highest maximum frequency, rather than a highest "FSB". Motherboards (or their chipsets, to be a little more price) generate the FSB, and thus are a more likely candidate for limiting the FSB. Don't forget that a number of top end Intel CPUs shipped with a default FSB of 1600MHz (i.e. 400x4), so the chipsets that support those must be able to run at a stable 400MHz 24/7, meaning they're likely to have some overhead beyond that for overclocking.