database tuning for high performance

Hello we have a large application which currently is using mysql 5 for data storage. Would like to talk to someone that is really experienced with databases and performance tuning, so we can make a choice for a new DB system and distributed architecture (we have around 4 servers now and planning to add 5 more). You must be proficient with MySQL / postgresql tuning, clustering, load-balancing and benchmarks. Before awarding the project I'd like to know more about how you are going to deal with this. I'll provide more details by PM of course. No dumb bids please. Thanks