I have only talked about the Hardware versions in ESX/ESXi product line. There are other products from VMware that have their own support issues such as the VMware Workstation and the Fusion product lines for hosted solutions. You have to really know what version of hosted product you have. For example, VMware workstation 6.0x supports Hardware version 6, but VMware workstation 6.5x and VMware workstation 7.x support Hardware version 7. I like that the hardware version numbers line up with Workstation product line versions: Workstation 7 gave us Hardware version 7, Workstation 8 gave us Hardware version 8 support, and Workstation 9 gave us Hardware version 9 support.
When moving a VM you must remember that the product you move it to cannot power on a VM with a hardware version that is higher than it supports. But if you move a VM with a lower hardware version to something that supports a higher hardware version, then it will power on, but some functionality will be lost. Therefore, my recommendation is to keep the versions in sync or not choose the latest and greatest hardware version until every host has been upgraded and every VM has been upgraded to that version of hardware.
I mentioned a few times that you can use the vSphere client from one of your administration machines and point that to your ESXi host to manage it. However, if you are not a small company, you probably have too many ESXi Hosts in your organization to make this practical. Well, the good news for you is that there is a centralized administration option called vCenter. Historically, vCenter has only been a Windows solution, meaning you install the vCenter Server product on your Windows-based servers. This was good if you were already a Microsoft shop and had another Windows license. Starting with ESXi 5, we got another option: the vCenter Server as an appliance. Instead of using the Windows version and having to use a Windows license, you can use this Linux-based appliance. There are some limitations in this Linux version, but I hope future versions of this product will give us all or even more functionality than we have with the Windows version of vCenter.
Windows and Appliance Versions of vCenter
Both the Windows version and the appliance version (Linux-based) of vCenter can be installed as a VM, but the Windows version can still be a separate physical machine.
Using the windows based vSphere client to point to the vCenter where all the hosts are being managed gives you a nice, centralized view. You can also use your existing Active Directory infrastructure to login and start assigning permissions for whomever in your organization can access resources. Starting with vSphere 5.1, you also have a vSphere web client, which is more feature-rich compared to the previous version in 5.0. Certain actions, like creating a VM with hardware version 9 can only be done using the vSphere web client, not with the Windows version of vSphere client. For now, we might be running both versions of vSphere client, but it looks like we will move to the web client so there is no dependency on Windows.
I hope this gave you a basic understanding of some of the concerns or planning points to consider when you get ready to deploy vSphere in your organization. This is just a glimpse into the things to consider for vSphere.
There are security considerations if you are a security person in your organization. There are networking considerations when you connect your ESXi host that is virtually networked to the real networks, such as Cisco switches. At that point you have to consider VLANs, trunks, and various other things. There are storage considerations if you are a storage administrator. Do you have an existing iSCSI or NFS or FC infrastructure? We can establish connectivity by any of these means to the outside world. Who backs up these resources that are on these VMs? Maybe you are the current backup person in your organization, so this comes under your umbrella. These topics have their own security considerations.
Reproduced from Global Knowledge White Paper: VMware vSphere Essentials