What do you mean by 'browsing users'?
On our system we have information on the number of distinct users seen in the last 5 minute period. For example the current number is 1103. (I don't think the total number of users makes any significant difference to load; it's the number of requests per second or whatever that really does.)
To handle our load we have four webservers, and our database server has a lot more RAM than yours (64GB I think). Our database is postgres (as per this silly video with lots of swearing).
That 1103 is a typical number for us whereas your 500 was 'max'. If you were expecting 250 to be typical, then one webserver seems to be a similar level compared to our system. Using only a single webserver has the advantage that it makes session storage a whole lot simpler.
A more specific metric that takes into account browsing habits (ie 'one user' who just looks at course webpage is no problem, 'one user' who browses many forum pages is a significant load) is the number of Moodle log entries per 10 minute period. We are seeing around 15,000 at present.
Note that our live system is still 1.9 - reports seem to suggest that 2.0 is at least twice as slow, so if you're using 2.0, you might want to consider increased capacity.
In general I would advise that you try to ramp usage into your system so that you have time to monitor performance and increase capacity if required (i.e. rather than everybody starts using it on the same day).
--sam
