Hello singhraj,
"i can add more Physical Ram if needed."
Yes, if you want to run NSX, multiple vCenters and vSAN hosts with the disk configuration and assigned memory you outlined you will likely have to go 128GB (or assign less memory to the hosts and go 96GB - though this may be tight).
"please advise and add you experience."
Sorry but your diagrams don't make much logical sense:
For starters, what do you mean by 'Data Centers' here? If you mean vSphere logical Datacenters (as you would have in vCenter inventory) then this would imply that you intend to create multiple vSAN clusters in different Datacenter/vCenters?
All members of a vSAN cluster have to be managed by the same vCenter and must reside under the same Datacenter Object.
You have "Node1 (cluster1), Node2 (cluster2)" - vSAN requires minimum of 3 nodes per cluster (or 2 data-nodes + 1 Witness).
Then you have "node3, node4" in another Datacenter with no indication of what these are clustered with (each other? cluster1/cluster2?).
If you do not yet have experience with setting-up/administrating vSAN environments I would strongly advise spending some time building (and dismantling!) some clusters using HOL:
https://labs.hol.vmware.com/HOL/catalogs/lab/4210
Bob
