48GB of memory is severe overkill. 8GB should do fine for four or five copies of WOW, and if you find that to be a bit tight, just double it to 16GB. RAM costs are rising a bit, but are still pretty low.

And I would also agree with building two less powerful systems as opposed to a single more powerful system. I don't know if you can do it for $2,200, but it's possible you can even do it for less depending on where you decide to make trade-offs. Just as a quick rundown of some prices (via Newegg):

Core i7-2600 is $300, Core i5-2400 is $185 (or $370 for two).
3GB 580GTX is $540-590, 1GB Radeon 5770 is $120 (or $240 for two).
128GB SSD can be as low as $150-200, 750GB 6Gbps/64MB 7200RPM SATA drive is $70 (or two for $140).

And so on. Again, it depends on how much you are willing to give up (you may feel that the 5770 just isn't going to cut it, for example). If you prefer a single machine (for reasons of space, or heat, etc) then a Core i7-2500K can provide the horsepower you need (and then some, since the "K" models are easy to overclock) at a savings of around $60.