If your vCenter is version 5.0 and you want manage the new ESXi 5.5 hosts and upgrade the old hosts to ESXi 5.5, you will need upgrade your vCenter first to version 5.5.
With vCenter on version 5.5 you will be able to manage host running 4.x and 5.x versions.
Here is the KB article showing best practices to upgrade the vCenter Server: VMware KB: Upgrading to vCenter Server 5.5 best practices