You need to define what you want to achieve
Do you want to play with virtualisation in order to learn about the guest OS's (the ones you will run inside the Hypervisor), or to learn about how to run virtual loads (i.e. how to use the Hypervisor) - if the former then you can pick any Hypervisor - either a built in to an OS one such as Hyper-V on Windows or KVM on Linux, or even VMWare Player etc. and just build a machine with decent resources - any 64 bit processor with the relevant virtualisation extensions will do. You can play in as littles as 4GB if you only need one VM at a time (or several tiny ones), but 8 or 16GB is more realistic.
If you want to learn about Hypervisors, then the choice depends on what you want to learn - for the corporate markets then it's probably Hyper-V or ESXi (which has a free version), possibly XenServer (also has a free version). The free versions are generally missing some of the high-end tools, but enough to get started. If you want to learn from a hosting/ISP point of view, then look at offerings such as CloudStack, OpenStack etc - there are development environments that will run on a single machine for test purposes, but these are generally aimed at a multi-machine setup (storage servers, compute nodes, cloud controllers etc.).
Or, if you have the time, and far better if you do, try all of them. Get to grips, work out which interests you the most and does what you're looking for.
Have fun. Here's a beer to drink whilst waiting for installs to complete!