Say I have a Dual-Core server, that's 4 cores w/ two physical processors.

I read numerous articles that states the dom0 should get one physical core to itself. By core, does that mean a single CPU core or one of the 4 logical cores? Ideally I would like to dedicate a single CPU core (2 logical) to the dom0. Then I would give the other CPU split between the 3 VMs. I've seen examples where ppl would assign more than the available number of cores to a VM and I don't know what good that would do. I mean, why would I want to assign 4 vCPU to a single VM when I only have 2 available (if my math is correct)? I assume I only have 2 available from the one core as I've given dom0 a CPU to itself.