Welcome to the Community - Essentials actually limits to you to 3 physical machines with 2 processors for a total of 6 physical CPUs - indeoendent of the number of cores in the processor - essentials also includes a copy of vCenter which provides a central spot to manage the the 3 ESXi servers.
Another choice you have is the free version but here the limitation is on the memory - with the free version you are limited to 32 GB per server.
With 512 GB I think you will have to look at the vSphere Standard license - which is licensed per processor - so you will have buy four licenses -
You can learn more about licensing here - http://www.vmware.com/files/pdf/vsphere_pricing.pdf