At what point do you need a 2nd server?
 

On average my board has 1200 concurrent users. This sometimes peaks to 1900. I see this is growing steadily.

I recently got a new server which is as good as it comes ATM. Dual quad core 2.2 Ghz with 15K HD's. All specs as high as possible.

My site runs well and fast. So thus far this is going well. But with how many concurrent users should I expect to be in need of a second server?

whenever performance is not acceptable anymore.

But then the second server should be there already. I am trying to be proactive and get an indication of when that would be.

That is impossible to say, too many factors to consider.

Just watch for it, it more then likely won't happen over night if your current server is holding up strong.

When you see the page load time steadily increasing everyday, and when performance starts to become "sluggish" put in for another server, hopefully it'll be ready in time and you shouldn't encounter a bunch of wasted hosting fee's :)

when you run out of money to buy the most expensive server to host everything on it and you still have subpar performance, thats the time you're ready to split your forum across 2 machines

I still believe sometimes is better to have a forum split into several servers, performance could be way better than having, lets say, a mainframe for the whole package.

my 2 cents