Just going to shed a bit of technical light on this to show how we do what we do here at my job.
Basically, if you have a beefy machine, and by beefy I mean strong CPU and lots of ram, a server could care less about video, thats primarily done client side, you can run anywhere from 4-8 virtual machines with ease, especially on a linux machine with little overhead, windows might give you 3-4, kinda depends on what VM you're running with Windows.
I currently use a development environment built entirely on VM's. I have about 6 file brokers and 3 database machines running off the same "beefy" machine. Its not even that beefy, its a AMD quad-core (2.5~ ghz) and 8 gigs of ram. But it is running 6 instances of Java and 3 instances of MySQL at any given time. This machine handles it pretty well.
the point of this is basically to say that Frostbyte has the right idea. If you want to setup your own private server, you can do it with VM's, running 3-4 instances to create a "solar system". Each solar system would then be able to register with a controlling server, i.e. tell me where you are, then people will be able to use mapping provided by the controlling server to request entry into a solar system, an admin could then say yes, add to the while list, or say no, peace out homie.
Now for the part that I think is cool, if we're able to setup a controlling server, i.e like a "Central FutureCraft Server", this also would allow us to establish what would in effect be a interstellar space market. Basically using a database, we could log, transfer, exchange and what not items and game currency. So for example, guy on planet 2 of Joes Crappy System needs Titanium, but his planet is so big he doesn't feel like looking for any, but because he traded a bunch of his iron, he now has a bit of money to spend, so he decides he wants to just buy a shipment of titanium, he'll be able to place a "request" for goods order, and if someone wants to sell some, they can place a sell or offer a big (i.e if the guy only wants to pay 20 credits they can counter and ask for 25 or something), then once a successful transaction is made the guy from Joes Crappy System will get a shipment of Titanium and the credits will be removed.
These are just some of the concepts I'm working out the details for. VMs would make inter system travel very easy (local IP's are much easier to find lol) and integrating a database controller with Java isn't rocket science.
Anyway, thats my thoughts on that. I'll be writting up and concepting this further, I have a machine at home I'll be installing a fresh copy of Cento's 6.5 or 7.0 or w/e version it is I have laying around, then I'll setup 2 VM's and see what I have to do to make MC talk to itself on different servers, actually, I'm going to google this first, I'm sure someone's already done the leg work on this one lol.