What are some rails app signs to step up your hosting plan?

my rail applications are currently on lighttpd, how do i know when i
should step it up to a mongrel cluster?

also noob question here: in the debug window, for every action executed
you see 37 requests per second or something like that. does that mean
that the particular action can be called 37 times per second? what
happens if it is called more than that per second from many different