How to guess approximate user numbers

Hi
I decided to make a site for football fans,
I want experienced users to Guess an approximate usage capacity of the system.
I want to have an idea that how many concurrent users can use drupal normally.

Here is the system i think from GoDaddy Hosting

A virtual dedicated server for drupal files
Centos
512MB Guaranteed Ram 2GB burstable

A dedicated server for only mysql database
Centos
Celeron 2.0Ghz
1Gb ram

I do not expect an exact number of user that can use forum, but on normal conditions within your experiences
how many users do you expect to such system?

(ie Can this system handle ~400 concurrent users ?
or let say ~100 pages per second to read?
or let say ~700 page submissions by authenticated users)

Kind Regards.