Apache+Mongrel to Apache+mod_ruby

I'm looking into mod_ruby because the Apache+Mongrel combination that
I'm currently using seems to have big overhead and somewhat slow, but I
don't quite understand a few things:

1)  If I move from the Apache+Mongrel combination to Apache+mod_ruby,
will my overhead decrease?  Significantly? (definitions are up for debate)

2)  Will I have to change any Ruby code?

3)  In regards to the "shared interpreter" problem: I run multiple Rails
apps on the same server; however, does this mean if I define constant A
in one app and then also in another app I will get errors and/or it will
be overriden?

Let me know if I need to clarify any question.  Also, feel free to voice
your opinions on the "best" Rails webserver setup (for shared hosting
environments).

-- 
  Travis Warlick

  "Programming in Java is like dealing with your mom --
   it's kind, forgiving, and gently chastising.
   Programming in C++ is like dealing with a disgruntled
   girlfriend -- it's cold, unforgiving, and doesn't tell
   you what you've done wrong."