I doubt anyone is running DC in their system.
Aside from the fact that it would be a complete waste of it's capabilities, it would probabaly be ALOT slower than Win2K Pro since its made for super high end servers.
DC is, in short, what Microsoft has produced to compete with systems such as Sun's E10K servers.
I dont think you can buy it by itself, it comes with some servers, such as Compaq's ProLiant 770, a server with up to 32 CPU's and 32 GB of mem(64 sometime in the future).
If I remember correctly, it doesnt even have a GUI installed by default.