Ah, my mistake.
You might be surprised at the audience that an indie MMO can pull in. Google "Minions of Myrth" and check it out. It was written using the Torque Gaming Engine and last I heard they had about 9,000 players or so.
It all depends on the content, and how fun the game is.
If you want to make an MMO, since you're already using GDK, which means access to anything that C++ can access, then you might have a look at RakNet for networking. It gives you all the features you could ever hope for in making an MMO. Multisync at the moment may be able to handle a few dozen players, but you'll hit it's top speed pretty quickly if you go over that. RakNet should be much faster, and gives you both TCP and UDP to work with.