I strongly advise you to do your homework before spending money on non-server class hardware (or before selecting a server for that matter). VMWare runs on a lot of hardware, but it also fails badly on a lot of consumer grade motherboards. There are some list (White Box Hardware Lists such as http://ultimatewhitebox.com/
) you can check. After spending some time on name server HW and on White Box gear, I can tell you that the name server gear is a lot more compatible, easier to work with, and worth the money (if you have it). If you are doing casual stuff and don't mind the considerable pain you will have to go through to get patches and select disks systems and other components, consumer gear will let you play a bit. As for doing anything serious with more than one VM on a box - not likely. Xen is a commitment, as is VMWare or any other VM system. It is going to eat the box if you do anything other than dabble in it, and you are going to spend some real money if you intend to do much with VMWare (think $3K - $5K to get very serious). Running a VM is easy. Running multiple servers, backups, external disk systems, etc. is real work and costs real money for all the extra stuff you will need. If you stick to Linux you can save a bunch, but if you intend to do any real work with MS Servers, you are going to need several licenses, and iSCSI targets, back up tools, etc...You won't actually learn too much before you go to that level that you can't learn with VMWare Workstation (a great product but not anything like a production server environment).
You can get you feet wet for nothing but time with most of these tools, but you can't get real, in depth experience with what it takes to run a production cluster, replication, remote storage, live replication, and all the rest of the things you need for real production unless you actually set up a production like system - that means real servers (White Box or name brand) and lots of hardware. You won't be able to see much with less than 8 cores, and 16GB plus some local RAID and iSCSI network targets. You can get started, but if you are going to spend money, I really think you should start to buy gear that is going to build towards a real server environment or you should stick to home systems and maybe run VMWare Workstation or some other stand alone VM just to play with it. VM Mode Linux (not very popular today) or some Xen sets for personal use would give you some understanding of VM concepts, but not a lot of basis for real production issues (at least they did not for me and I was a pretty heavy development user). Production VM deployments have a lot of issues that all take real in depth study, and lots of resources (iron) to get right. On the other hand, you can get a Supermicro, a Dell, or HP server with dual Xenon quad cores for less than $4000 with some nice disk. Get 4 or 5 containers under a VM and set up a replication to another server and a remote iSCSI disk and then you have enough to start to actually do real learning on. Of course the license fees will be way more than the hardware costs, if you are using MS tools and VMWare. ESXi is OK but unless you are going to go deep and do it all the hard way (hack the OS) you can't do a lot with the free version. With Xen, if all you want is to run a couple versions of Linux, just get a quad core box and have some fun...it doesn't really give you much production knowledge, but you will have some interesting test you can try.
What I am really saying is - with only 4 cores you can do some useful things to support development,and you might make a nice personal server for you private web sites, but you don't have enough iron to experience the real issues of production VM management. If you are going past what a developer does (or a tester) and looking at a operations type environment you will need 8 to 16 cores on multiple boxes. This is a lot more than a home user typically wants to spend. IMO you also can't really expect to be really good on more than one system unless you do it day in and day out. I know that there is way too much to learn about VMWare to do more than that as a full time job at if you are doing serious production work. This is OK, as jobs with any one of these are going to be pretty highly paid and intense - there is only so much you can stand...
Bottom line - you can start with a 4 core consumer system, but you can't do much more than get your toes wet on that hardware. Size and money count in this game. You are building nothing less than the equivalent of a mainframe - don't expect it to be easy or cheap. You can learn to do valuable stuff as a single user, but learning real production configuration is going to take real production hardware. If you are going to buy server gear, go ahead and buy the real thing - not some game PC. There is a big difference.