Go through the documentation to this forum http://docs.moodle.org/en/Performance first.
Then look for similar discussions here, for example "A moodle setup for 10K simultaneous users" http://moodle.org/mod/forum/discuss.php?d=111847 you can use the search facility built into Moodle.
Hardware and performance
Specifications Moodle Server 8000 user
This discussion has been locked because a year has elapsed since the last post. Please start a new discussion topic.