Ok, I have just confirmed that the new CPU family that I'll have on my ESXi 5.1 host servers is from the Sandy Bridge generation which vCenter 5.1 supports as an EVC baseline, but the ESX 4.1 host only support up to the Westmere generation which is the generation before it.
My VMs (if they take on the EVC features of the host/EVC cluster they come from even if they are going to be stand alone in the new vCenter 5.1 will not allow me to vMotion them into my ESXi 5.1 host cluster using the Sandy Bridge EVC baseline...correct? I would have to use the lower "Westmere" EVC baseline in order to use vMotion or do cold migrations...correct? I'm sorry for these "am I correct" questions but just want to be sure even if I'm 99% sure this is correct 
I guess even if I used the "Westmere" EVC baseline, I could always change it to "Sandy Bridge" EVC baseline after all VMs have been upgraded to new VMware Tools & Hardware 9.
Getting closer to having the planning phase completed for this upgrade/migration to vSphere 5.1, so thanks for all your help! 