1,500 concurrent users... eek! I'm guessing you know this already, but that server won't come close to handling that load.
You'll need several / a cluster of front end webservers, a well spec'd DB server and some from of Data storage accessible to each web server (for the moodleData directory). Exactly how each of these systems needs to be spec'd up depends on exactly what your courses will be serving, for example will it be serving lots of multimedia content (serving large files of this type is hard work for webservers which tend to be better at serving lots of small files) or will the site be largley text based. Will the site be very interactive (there are plently of discussions on these forums about the amount of resources consumed by Quizes, Chats erc) or is the content largley static (to be read / downloaded only?)
A moodle installation of that size sounds like quite a large project to take on, I'd suggest talking to a
Moodle Partner about your installation, or at least getting in contact with people with experience supporting large Moodle installations.
Best of luck with the project
