If you have VirtualCenter 2.x and you want to manage ESX 4.0/ESXi 4.0, you must upgrade the VirtualCenter software to vCenter Server 4.0.

ESX 4.0 and ESXi 4.0 hosts that are managed together in a vCenter Server system can run the same virtual machines, use VMotion to migrate virtual machines between the hosts, and access the same datastores.

You can manage ESX 3.x/ESXi 3.5 hosts in the same cluster with ESX 4.0/ESXi 4.0 hosts. You can manage ESX 2.x as standalone hosts in a vCenter Server system. ESX 2.x hosts cannot be added to clusters.