Paul Burdick at Solspace has written a totally awesome document about EE Performance Guidelines. With his permission, I'm sharing it. Please contact Paul or Solspace directly for updates or a site evaluation.
While I'm on the subject, here's the two add-ons currently in Solspace's new performance suite. This includes Morsels -- a new DB-based acceleration method, and Page Caching, a method to cache entire pages as flat files.