very little required, depending on the workload (Score 1)

If your intent is to educate yourself on the various configuration options, then the workload of the "applications" (Oracle DBMS, etc) is modest if it's idle. I have configured a Oracle RAC two node cluster using a pair of basic notebooks and a WD MyBook World Ed NAS, which is pretty weak hardware and low cost. An upgrade would be a couple of PC's with CPU's that support virtualization, such as KVM, like Intels VT or AMDS AMD-V class and a NAS. You could even use FreeNAS, which I recommend regardless, for shared storage. VM's are generally constrained by memory. There is also the option of renting cloud resources, ala Amazon EC2 or Elastra et al, which is probably very cheap considering the light workload and data required. As with most things, it depends on what you want to accomplish. Consider the minimum requirements of the packages or applications and factor for virtualization overhead. Obviously, performance tests require much more resources.