I need performance help on my wordpress installation

I am really looking for people to recommend others, not themselves. Who are the experts in performance tuning for a WP install running on two dedicated servers? Apache/MySql/eaccelerator/Zend/caching in place. Site average gets about 500,000 visits per month and is just too slow and I need to find the bottlenecks. Please give me a reputable expert to contact, I appreciate your help.