Assuming you've got control of the server, getting it to run under mod_ruby will be faster than either fcgi or webrick. If you're not willing to go that far, yeah, fcgi is probably better than webrick (judging from the tiny bit of experience I had with an older version of webrick).
Also you should be able to just change webrick's port to 80 but I'm not into all that rails stuff so I don't know where that particular configuration would be located.
As for mod_rewrite, from
the docs:
Code:
# For sites running on a port other than 80
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{SERVER_PORT} !^80$
RewriteRule ^/(.*) http://fully.qualified.domain.name:%{SERVER_PORT}/$1 [L,R]
edit: although this doesn't remove the :3000 from the visible url it does allow accessing the site without typing it in.