Evening,
The links above are great. A few things to consider are the following:
1. New Hardware version
2. New VMFS version (in place upgrade of current datastores are not the same) If you have the storage bring new storage in make it a datastore with 5.1 then migrate to it
3. New VMware tools
4. If you are using VDS then new version of the switch
5. You may want to look into Storage Clusters and Storage DRS.
A logical Plan if you had storage to add might look like this:
1. Maint mode a host
2. Fresh load ESXi 5.1
3. Use host profile (if enterprise plus) to make new 5.1 as equal as possible
4. Manually check new node for all networking etc..
5. Take out of Maint mode
6. Rise and repeat on all nodes
7. Add new storage version 5.1 and make it a storage cluster with Storage DRS enabled
8. Storage VMotion vm's to new storage cluster
9. Upgrade any VDS's required
10. When possible take outages on vm's to upgrade the hardware version and vmware tools
Please let me know if this does not answer your question.
Thanks