Mysql scalability and failover...


I am an owner of an large community website and currently we are having problems with our database architecture. We are using 2 database servers and spread tables across them to divide read/writes. We have about 90% reads and 10% writes. We use Memcached on all our webservers to cache as much as we can, traffic is load balanced between webservers. We have 2 extra servers ready to put to use!

We have looked into a couple of solution so far:
Continuent Uni/Cluster aka Sequoia -> Commercial version way too expensive and Java isn't as fast as it suppose to be.
MySQL Proxy -> We couldn't find any good example on how to create a master - master with failover scenario.
MySQL Clustering -> Seems to be not mature enough, had a lot of performance issues when we tried to go online with it.
MySQL DRDB HA -> Only good for failover, cannot be scaled!
MySQL Replication -> Well don't get me started ;)

So now I turn to you guys to help me out, I am with my hands in my hair and see the site still growning and performance slowly getting to its limit. Really need your help!! HELP!