But you maybe need a powerful database server to save the stuff your users will do. It's normal to log everything that happens in an MMO game, regardles if it's useful or not. So you should buy some SSDs and a few gigs RAM (around 8 or so, to avoid having I/O operations on the disk).
Shitlord by trade and passion. Graphics programmer at Laminar Research. I write blog posts at feresignum.com