I'm trying to install Rails 3 on a brand new MacBook Pro running OS X 10.6.3, Ruby 1.8.7, and Rails 2.3.5 and I'm wondering if I've hosed myself. So far, I've run these commands:

$ gem update --system
$ gem install arel tzinfo builder memcache-client rack rack-test rack-mount erubis mail text-format thor bundler i18n
$ gem install rails --pre

However, when I run the last command, I get this error:

ERROR: While executing gem ... (Gem::FilePermissionError) You don't have write permissions into the /usr/bin directory.

I think it wants me to run the command with sudo so that it can write over /usr/bin/rails. But if I do that, won't I be overwriting my Rails 2.3.5 executable? I don't want to do that. Instead, I'd like to keep both versions of Rails. Can anyone help?

Thanks.