Depending on how you setup your disk (ie multiple spindles or SSD) you could put a LOT of virtual machines all on the same physical box. The CPU itself doesn't matter too much although I'd recommend picking something with 4 cores (not HT). The machine would easily run at least 4 VMs. That way you could have 2 DCs to practice setting up a site plus another running WSUS. Most "desktop" motherboards these days can go up to 16G of ram. Since you're just using this as a learning tool you could prob get away with using 1-2G of ram per virtual server. I will emphasize all these recommendations are for a box that performance and reliability are not the most important requirements.
If you are truly "green" you might way to consider first just installing Server 2008 R2 Standard and enabling HyperV on it. That way you can get familar with the concepts of virtualization with the luxuries of doing it all on a windows machine (managing hdds for instance would be much easier). When you are ready to get into more advanced virtualization you could put ESX, XenServer or HyperV server on the physical machine.
