I think it depends on how you're defining performance, and if you were experiencing poor performance before. Each core is treated as logical CPU, and thus independent as it relates to the vmkernel. The big area where multi-core processing is important is in the overall workload of your ESX host. The vmkernel is now going to be able to distribute over 4 physical cores instead of 2 physical cores (and two logical ones if you're using an Intel proc with HT enabled).
I am not CPU bound on any my VMs, so I should be fine. It was just something that I thought about as I was monitoring the new dual core server. Something I never thought about when I bought the dual cores.
So it sounds like dual core has the capacity to run more machines, and single core generally has higher maximum CPU perf. for a VM.
One thing that is cool is that I have noticed my baseline CPU utilization has gone down 40+% going from single 3.0Ghz to dual core 2.8 ghz. It really seems like i am getting near quad CPU performance from my 2 dual cores.
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If your CPU load isn't heavy on the host you might be ok running a vSMP on a 2 dual core box. Personally I find that half of the time you get slower performance on a vSMP VM. Lets say you have a VM configured as a dual processor system. The way vSMP works is the VM must wait for two processors to become available to do any processing. So if you were running on a 2 way single core system, the VM will be waiting a long time to get any processing done because there are only 2 CPUs. On a 2 way dual core you get 4 CPU's so now you need to wait for 2 of those CPUs to be free. The vSMP VM's speed will increase with more CPU's in an ESX host. So if you have a decent amount of VM's running on your server the vSMP VM may not perform well (and may actually be slower than the same VM configured with only 1 CPU).