From talking to many individuals who are wishing to achieve VMware certifications such as VCP or VCAP-DCA, most people have to prepare for exams in their own time. One of the important elements of both these exams, particularly the live-lab VCAP-DCA, is to use a vSphere environment to practice using the various administrative tools and interfaces in readiness for the exams.
Some people are lucky enough to have access to a lab environment at their workplace that they can "play" on, but many inviduals end up trying to build their own home lab. This can be done either be using a number of separate physical machines, or building a vSphere lab in virtual machines on products like VMware Player or Workstation, or even the free edition of ESXi.
Whether physical or virtual, generally there are four systems required:
2 x ESXi servers (ESX can be used instead)
1 x Windows server running vCenter Server, SQL Express, and the vSphere Client
1 x shared storage server running FreeNAS, OpenFiler, or similar
There are already many blog articles out there on how to build a virtual vSphere lab:
Here's a very useful site that deals with the software/build process and offers a recommendation on hardware too:
One of my students this week has just told me about an amazing deal for building a virtual home lab system, it's a HP ProLiant MicroServer on sale from EBuyer, with a £100 cashback offer - as of today the effective price you pay is only £137.99: http://www.ebuyer.com/product/253305
You will need to upgrade the RAM to 8GB in order to run the required virtual machines, but that's only going to cost an extra £64: http://www.ebuyer.com/product/229102
Of course, you might want to upgrade the hard disk to something like an SSD drive for better performance, but that's up to you - the 250GB built-in disk should easily be big enough to accomodate all your vSphere VMs.