Just thought i'd add the high level steps as I see them:
1. New vCenter Server on a 64-bit OS – Windows Server 2008 R2.
2. Latest version of vCenter 5.x – retaining compatibility with ESX 4.x Hosts.
3. Retain hostname and IP address of old server.
The plan to achieve these objectives are as follows:
1. Build a replacement vCenter Server virtual host on a 64-bit OS
2. Create a backup of the existing database
4. Create a backup of configuration information such as port settings, SSL certificates, and license information, using the data migration tool on the vCenter 5.0 installation media.
4. Shutdown the existing vCenter server, “transplant” hostname and IP address to new server.
5. Restore DB backup to a new SQL instance on the new server
5. Using the data migration tool, install vCenter Server 5.0 on the new server, point to application to the new SQL instance.
6. Perform an inplace upgrade of vCenter 5.0 to 5.1
6. Instruct administrators to install the new version of vCenter Client.