Vcenter query, Upgrade 4.0 to 5.1

We have 2 esx 4.0 cluster and want to upgrade to ESXi 5.1 + a physical server running Vcenter

I want to add a new ESXi 5.1 host to this cluster and then move vm's and upgrade other 2 host as well

How should I begin?

1. I know my host servers and ISCSI SAN is in the compatibility list

2. Upgrade V-center to 5.1, however following forums says some issues:

http://communities.vmware.com/thread/428382

3. add new ESXI 5.1 host and go.. on

So should I first upgrade V-center to newer version, other upgrades will another 2-3 weekends

Will new v-center work with new and old host simultaneouly?

or can there be a better plan.

Thanks