http://www.devside.net/articles/apache-performance-tuning is a good article on this topic.

In a nutshell, here are things you can do to reduce memory foot print:

1. Review the list of loaded modules, and remove the ones that you don't need or use
2. Change mpm_prefork to mpm_worker
3. Limit MaxClients in mpm config.

First, please check the Apache website for

• Apache HTTP Server Version 2.2 Security Tips 
• Apache HTTP Server Version 2.0 Security Tips 

http://xianshield.org/guides/apache2.0guide.html has some nice tips on how to harden your apache server. Especially, check your httpd.conf and make sure the configurations are fine.

Here are some of the ways to run Ruby on Rails behind Apache.

Apache + mod_fcgid

Reference Howto Setup Apache With FastCGI And Ruby Bindings.

Apache mod_fcgid

Make sure mod_fcgid is available, if not, install it
# emerge -pv www-apache/mod_fcgid

Ruby fcgi

I was not able to install it via gem. See error below: