my understanding of ratios is having your CPU FSB higher than your memory speed will offer no advantage whatsoever. you want to have at least a 1:1 ratio.
another thing holding you back could be the NB strap. i think as soon as you go over 400 FSB, you're in a new strap and will have higher latencies. so trying out different frequencies might yield interesting results.
maybe loosen your memory timings to see if you can get DDR 900 speeds? you've already demonstrated that you can get 450 FSB with a very low V Core.