The biggest problem you're going to have is with table locks, especially in the members table. If you haven't done so already, switch from MyISAM to InnoDB, which should help a bit.
Threads can be properly served in a clustered environment, but it's only written to think about talking to one database server, and that's where your bottleneck is going to be anyway. Configuring any sort of MySQL cluster is beyond my personal experience, however you might have some luck looking at MySQL's new "MySQL Cluster" product.
There are also some other options available, and I'll write you in a private message for those. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />