Apologies for this being in the wrong place.
Thank you. I also checked out ftp://ftp.dell.com/Manuals/all-products/esuprt_electronics/esuprt_software/esuprt_virt_solutions/vmware-esxi-4.x_Reference%20Guide2_en-us.pdf for ESX4 and all appears well, but I will need EVC mode on.
I assume, once EVC mode is turned on, it will always be set to the level of the 'lowest' (oldest?) CPU level (in this case Nehalem) until the last host with Nehalem CPUs is retired from the cluster.
Thank you again.