To begin with, when considering a vSphere Private Cloud Cluster or Private Cloud Hosting deployment, there are two basic areas where you need to assess your requirements; virtual machine resource capacity and vSphere features such as High Availability (HA) and Fault Tolerance (FT). Taking into consideration the immediate needs, you also need to consider what they will be in the future as a small additional outlay now could save you from spending a much bigger sum a year down the line. One of the key benefits of virtualization is that you can separate your software upgrade cycle from your hardware upgrades, so you don't have to worry about whether you should update your server OS when you virtualize them. You do however need to consider what the resource requirements will be if you undertake a software upgrade cycle 18 months later, as new software almost invariably requires more resources than its predecessor.
Determining Hardware Requirements:
There are two important factors to consider here, first of all vSphere licensing makes it desirable to use high power quad core CPUs, and the 'High Availability (HA)' feature requires the hosts to be almost identical.
Minimum Hardware Requirements:
1) ESX Host Servers: If you wish to use 'High Availability (HA)' or 'Fault Tolerance (FT)' between your servers, you won't be able to use ALL the resources available on the host servers. That is why VMware recommends minimum of 3 nodes in a cluster instead of 2, because then you can have quorum reads + writes and still have both, strong consistency as well as availability in case any single node goes down. With just 2 nodes, you will need to choose between strong consistency (write with ALL) or availability in the face of a single node failure (write with ONE) but not both.
2) VICENTER server: The Private Cloud Cluster also needs a separate system to act as your Virtual Center server, which manages and controls the ESX host servers. For a small vSphere Cloud of less than five host servers, a minimum configuration system can suffice for this function as long as it runs at least Windows XP Pro and 2GB RAM as recommended.
3) Storage Server: When it comes to specifying the storage for your vSphere Infrastructure, the first decision to make is whether you would require a Storage Area Network or not. Storage Area Network (SAN) is essential if you wish to implement High Availability as that requires the VMs to be stored on shared storage available to all the host servers. Even though there are a few other storage options available, we recommend SAN Storage as it is capable of meeting the IO requirements of the VMs.
Recommended Configuration: You will at least require 5 servers (1 VIcenter + 3 ESX + 1 Storage) to setup a private cloud. If you still want to reduce the cost then you can use a VM (hosted with another environment) for VICENTER.