5
When I said bandwidth, I didn't mean transfer limit, I meant actual bandwidth.
Let's say you get those 400.000+ users - even if "only" 10.000 are online, with 10 Kbit/s used for serving pages for each user, you would need a 100 MBit connection. If each webpage is around 40 KByte (not unrealistic), it would take up 400 MB of transfer just to serve that one page per user. If these 10.000 users are online at the same time, it would take 40 seconds before they had all received the page from the webserver.
With that math, your 2 terabytes will be over and done with after 50.000 clicks - i.e. 5 pages viewed by each of 10.000 users in a month... or 1000 users viewing 50 pages - and if your content is good and your userbase is half a million people, that is not at all a high number.
Good luck - If your user calculations are correct, you are in for a real treat in how to run a high-performance webserver, because that will be needed
*crossing fingers for you.. and XOOPS*