I haven't seen any updated numbers for 5.x, but this chart for ESXi 4.1 indicates that a VM with 8 vCPU and 256GB memory needs over 10GB extra memory reserved for overhead.
VMware vSphere 4 - ESX and vCenter Server
It looks like the requirement is not quite double when you double the allocated RAM, and a large increase as you double the vCPU count as well.