For Perenthia I am planning on utilizing server memory as much as possible without crippling the web server. To do that I am going to be maintaining player data and room data in memory for fast access and quicker responses from the interface.
The basic idea is that players login, choose a character and begin playing. This character information will be loaded into memory and persisted to the database when the data is changed, such as when commands are executed. As players move around the rooms they enter will also be persisted into memory to make loading and re-loading rooms faster.
The trick will be effective management of these resources. Character data will be removed after inactivity but I haven't figured out a good scheme for removal of the room data. Hope to have that working before too long though.
Anyway, that is the plan, we'll see how it works out. :)