Another option is to build new. I went with this route when I went from 4.1 to 5.5.
We built new new vSphere host at 5.5 using an extra host we pulled out of the cluster. We then deployed a new vCenter server. We then removed another host, rebuilt it using the 5.5 ISO and joined it to the new vCenter in the cluster with the 1st host. We then removed 1 host at a time from the 4.1 vCenter and joined them (live) into the 5.5 vCenter, in the same cluster as the 1st 2 (making sure to have DRS & HA off). We were then able to migrate the VMs to the 5.5 hosts with 0 downtime. As we moved VMs, we rebuilt the 4.1 hosts using 5.5, until all hosts were at 5.5.
I usually prefer this method as you're getting a clean install each time.
However, like TheBobkin said, you'd need to go to 5.x first before going to 6.5.