on a p4s533 the 1.6a would have the advantage.
it would be better suited to run 160fsb 4:5 400DDR.
a 1.8A most likely wouldnt make it that far, but at least the 1.6A has a chance.
even if the 1.6A couldnt do 160fsb, it again would have a better chance at 150 or 155fsb.
HTH