When to add mongrel clusters

I am running a website right now on one mongrel. Things are going well
at present, but of course I expect the site to grow and need to know
when, how, and perhaps IF i need to add mongrel clusters. Isn't adding
mongrel clusters the way to scale a RoR website? Is there another way
that I don't know about?

You may wanna take a look at mod_rails (http://www.modrails.com/),
much easier to configure and the performance appears to be as good or
better for most apps.

It's never too soon to move to a high performance app server;-)