Adding another cpu

Hi,

I chave an esxi 5.1 server with 1x quad core cpu inside.

I want to add another quad core so i will have 2x quad core cpu inside.

I rang vmware and the lady told me i should be able to do this with my free license.

However, my license states: Product: VMware vSphere 5 Hypervisor Licensed for 1 physical CPUs (unlimited cores per CPU)

I will be having 2 physical cpus so how will my license cover that?

If i need a paid license which one do i get? I only need the features of the free version, just want an extra physical cpu?

Thanks!