[Ted S]

A 2 server setup is fairly simple... right now your forum is connecting to the local server, when you add a second machine for the database, your forum connects to that over an internal ip address. Your database server is just like a local database server, it can serve one db or twenty, although most of the time if you're splitting them out it's because you want more dedicated resources and don't want to be sharing them.

When do you do to this setup? When you need it either because you can see a slow down in services or because you forecast one (and hopefully not because your site is crashing from overload although that's often the case).

The number of visits and pageviews is telling but not the whole story. Sites running a barebones installs with short sessions might be able to do two, three, even four times the traffic of one with more features, addons or visitor activity. If your site runs well at that level there's little reason to make a change... if it doesn't then you should look at optimization options and if those are maxed out, a second server is probably the ideal solution.