memcache doesn't necessarily speed up your Drupal sites
Sites that show up quickly on a user's screen tend to keep the user's attention for longer and also rank better in Google. So when listening to the excellent Lullabot Podcast 80: Top 40 Drupal Modules Revisited I was caught by Drupal experts stating that Memcache would even speed up your site if it's not a high traffic site. It could off-load the database. I decided to give it a try on a relatively low traffic site of one of my clients.