First I want to say thanks for answering my last question on nginx, very informative. My other question is what methods you guys recommending for keeping database servers in sync (primarily postgresql and mysql). I know different organizations have different setups and different sql platforms have different built in mechanisms  but I was wanting to know what you recommend for the following scenarios:

1) For load balancing, if you have 2 separate sql servers, how do you keep them in sync, so that if a user hits one server and update information it is updated on server 2. What if you have 500 servers. What would prevent a large delay?
2)If a database is either too large or would have too large a load is it possible to break up parts of the database over multiple servers, or must the whole database be stored on each server.
3) Would you recommend incremental or similar to scenario 1 sync for a failover server.