Advice on Building a Robust Load-Balanced Website

We are running an "auction-like" website that requires a number of ajax requests every n-seconds for each connected user/guest. There is also an application (java) server that is connected to the website's database. We are running it using 3 dedicated servers, currently using Galera Mysql master2master replication (3 nodes in Centos ESXi VMs each) and 3 apache webservers (again as a VM, one on each dedicated server). In the front there is a HAPROXY and an NGINX SSL terminator. At the moment we are dealing with more than 200 concurrent users, but it seems that DB performance is sometimes getting too slow, especially on insert and update queries. We are working on improving the DB queries and we are looking on buying more resources. We are also utilizing apc and memcache caching and all our static content is served via AKAMAI CDN. We need an advice on the required hardware, possible virtualization architecture and Apache/Mysql configuration tips to improve performance. We are also checking the possibility of moving to the cloud (rackspace or amazon) but maybe there are limitations due to the DB master to master replication requirements. At the moment we need a short technical proposal (preferably by an one hour skype meeting). Thinks to be discussed: 1) What OS to use (we are using Centos 64 bit, is it OK?) 2) How many dedicated servers do we need to serve more than 1000 concurrent users with mass Ajax requests (about 3-4 ajax requests every 10-20 seconds) 3) Do we need virtualization? What are the best practices and requirements of an apache webserver? Is it beeter to use one dedicated server with two apache servers? 4) How to improve our Galera infastructure? 5) is it better to move in the public cloud?