Typically you only have 2GB of Ram in a server any more than that is over kill. It is not processing videos or playing games. Increasing the ram cna help in seek times and also improve completing the required tasks that users are asking it to do. Servers just store information or manage websites. Having more ram enables the server to perform multiple user requests without lagging.
depends. Database servers at my work typically have 16GB-32GB of ram. Caching data from the harddrive into memory helps with response times. I've seen some large database queries run over 100x's faster on a machine with 8GB of ram instead of 4GB all because memory is thousands of times faster.