It's also worth spending some time trawling through previous discussions at this forum.
The trick is with "simultaneous" (or concurrent) - most admins here seem to use the number that are listed in the "Online Users (last 5 mins)" block. 500 is a fairly serious number.
Even without running the script to which Visvanath pointed, you can be certain that your intended system (2GB, single CPU for front-end IIS box) will not
support 500 concurrent users.
Most of the sysadmins contributing to this forum are also running on Linux. It might be worth having a chat to the folk over at the "Windows-based servers
" forum to discuss your planned specs.