Simple, OC the CPU, then you OC the ram
The Z7S currently only offers a 1:1 mem to FSB divider, so the only way to get your FBDimms up to speed is running the CPUs overclocked. Which you should do in any case
With a 99,9% probability a pair of E5420's will run FSB400 fine with all voltages set to minimum (you can't adjust Vcore anyways).