I have 2 ESXi 5.1 boxes, both Dell PowerEdge T610's.
One has Xeon E5620 and the other has Xeon E5520.
When I try and turn on FT, I get the message "The cpu pair is not compatible for fault tolerance: CPU model does not match".
Does it have to be the exact same CPU type to enable FT?
FT is much more picky about CPU models the vMotion and HA, so I would say that yes you will need the same CPU model.
You should use EVC mode on your cluster on this regard, EVC mode allow your virtual machines to use some of standard CPU features family and when you have two different CPU on your hosts, you have to check your old CPU kernel name and configure EVC according to that. New features on your new CPU will not accessible by your VMs and vMotion, HA and FT will be enabled for you.
Select "Nehalem" and check again.
No need to restart.EVC applying on cluster.
Not 100% sure but I don't think that EVC can solve CPU Compatibility for FT.
Your CPU:s need to match for it to work.
Are you also aware of the 1 vCPU limitation with FT?