Using memcache for me personally and the client sites I work on, has improved performance by over 1000%, in some cases even higher than that. It is a truly revolutionary way to optimize the performance of MySQL queries, remote requests such as to twitter or other apis, and this article is step 1. Plan is to go all the way to step 10 in future articles, it it grossly underused and misused in the WP world.
To prepare for several upcoming articles on AskApache that are focused on optimizing Servers and Sites from a server admin level, here is an article to introduce the main tools that we will be using. These tools are used to optimize CPU time for each process using nice and renice, and other tools like ionice are used to optimize the Disk IO, or Disk speed / Disk traffic for each process. Then you can make sure your mysqld and httpd processes are always fast and prioritized.