Moved to phpBB Discussion.
Here's something to think about... 100 concurrent users is a lot.
I manage a large board with over 20K members and we average about 100 concurrent users online during the busiest parts of the day. The average online users for the past two weeks for this board was 86, the minimum was 7, the maximum was 368. Over four weeks I had an average of 83, min of 7, max of 480.
Since I've been keeping track the average is 71 and max is 1,805.
What is this leading to? Two things.
First, in order to average 100 concurrent users you need to have a much larger userbase. I don't think it's typical to have 100 users concurrently online with a total memberlist of only 1000 users. The numbers seem to run about 0.5% to 1% of your total users online at a given time. So that means 10-20K total users.
Second... I run on a dedicated server. 100 concurrent users means you have a decent number of HTTP and database requests being made. In order to handle those you need - as has already been suggested - lots of RAM, and faster disks help as well.
If you are truly expecting 100 concurrent users right away then you probably need to size accordingly. The big board I mentioned did not start out that big, of course. It's nearly five years old at this point. For the first year we used a shared hosting account. For the 2-3rd years we were on a dedicated server, but a very cheap one. It was about $70 a month for a single cpu celeron box. Now we're on a dual Xeon cpu with 4gb of RAM.