VMware 4 upgrade and new host server - recommended options and order?

We have a VMware vCenter / ESX 4.0 environment with a couple of VM hosts. We plan on upgrading to 5 or 5.1 very soon, but we'll also be adding a new VM host and a new vCenter server.

It would be nice to have the 3rd VM host online before the upgrade so have the additional capacity in case there is any trouble upgrading. But I'd just assume avoid the trouble on building another 4.0 host to immediately upgrade.

We also need to upgrade the vCenter server and move to a new box. Still trying to decide between VM or physical.

So I was looking for a little guidance on the order to do things.

Should I bother building the new server as an ESX 4.0 system and upgrade it or just build it as a 5 / 5.1 box from the get go?

Should I keep the vCenter machine as a physical box or go virtual? I hear lots of pros and cons about both. For us it's been useful to be able to access vCenter when everything else is down. But of course it needs to backed up separately etc.

What order should I do this? I'm assuming something like ... build, migrate and upgrade vCenter, Build the new VM host, upgrade the existing hosts.

Thanks for any advice!