I've always been interested in maintaining performance on my websites and prepare for increasing usage over time. However, I could never find much information regarding how often is TOO often when it comes to querying a database. This is probably because there are several variables involved. Does anyone know of any articles where I can get better insight into maintaining performance, calculating performance, or creating databases that can handle hundreds of concurrent queries?

Mysqlperformanceblog.com is a good one, any others?