I'm not saying Michael P is wrong, but the project I'm working on right now is also a space based MMORPG. For the moment I'm using DBPro with MultiSync for both the client and server. It will be scalable up to a point with the server architecture I have in mind.
The basic idea is a Master server, and several secondary servers. All the servers have their own connection to the MySQL database for READ only access. The secondary servers connect to the Master server as special clients and relay their information about their client connections, and any information that needs to be updated in the database. The Master server accepts client connection requests, and then directs the client to the secondary server with the fewest connections providing load balancing.
So the Master server is the only one able to WRITE to the database, solving any problems with multiple WRITE requests. It also relays any information it receives from one secondary server to all the other secondary servers. The secondary servers will only send the updates to the clients if that information pertains to something within a certain distance of them, so unneeded information isn't transmitted.
There is of course a point where this configuration will become saturated, but it should be able to support a few hundred clients. As long as they don't all arrive in the same town at the same time.