I just checked Vmware's website for 1 socket (1S) performance on virtualiztion workload and could not find any publications on 1S - only 2S and 4S or higher.
So here is some data on integer perfromance (SPECint_rate 2006) where i have some downbin data that i can offer as a references to help you
two 3.0GHz Xeon (Xeon X5460): score 114
two 2.3GHz Xeon (Xeon E5410): score 94.4 (18% lower than X5460)
two 2.0GHz Xeon (Xeon E5405): score 86.1 (25% lower than X5460)
We have data that shows the benefit of adding a 2nd CPU to the same configuration server with SPECint_rate 2006.
Adding a 2nd X5460 to a platform running one X5460 is a 75% performance gain. So assuming that a E5410 and E5405 would see similar improvements (this is my personal extrapolation and not based on any measurements or publications available today.
one 3.0GHz Xeon (Xeon x5460): (personal estimate would be 65 - extrapolating that 114 is 75% higher than this number)
one 2.3GHz Xeon (Xeon E5410): (personal estimate would be 53 - 18% below above 65 estimate (frequency scaling of )
(personal estimate would be 54 - extrapolating that the 94.4 number
so i think in summary, using these estimates and SPECintrate2006
two 2.0GHz ==> 86.1 (measured data)
one 2.3GHz ==> 53-54 (pesonal estimate - logic above)
I suspect that the details you see will be different because your workload is not SPECint but this is the best proxy i can give you to help you make your decision. one final thought, since most users are building production virtualization environment with more resources (CPU, memory, I/O), i think that a 2S platform may be a better approach to model in a test environment.
Hope this helps.