I'm assuming you mean 10,000 - 15,000 otherwise it's quite a broad range of users you're planning for.
Anyway, I don't run a huge site, but my instincts are a little different. For sure get as much RAM as your server and/or budget can handle. But you might want to look for other bottle necks. You didn't specify, but SCSI hard drives make a big difference. As does tuning your mysql
, php, apache, dns, network, gateway, router, firewall, etc., etc.
With that many users, it wouldn't take much for your site to get "slashdotted" at the start of the term, or if you launch a new course or something, your server could get seriously crunched. Then all the reputation, acceptance of Moodle and/or your content that you've worked for will get a black eye.
I'd really make a worst case (or is it best case - you want a lot of users) assessment of how much content (html, scripts, graphics, media) bandwidth, CPU(database queries, php execution) and RAM(session data, database queries, php) your server will be asked to deliver at peak times and then make sure there are no bottlenecks anywhere in the system that preclude you delivering it.
With that many users you should consider your backup/redundancy plan very seriously as well.
Just my US $0.02.