Read the script, it's adjustable. Download, change your version and run. They are listed as they help 'cover all bases' as they are used by some gems that people might need.
Here is a script that I wrote and have been using, should work on most Linux's and OSX to bootstrap a Rails Development environment. Also ensures that git is installed. You can run it with one command (after reading it's contents of course) as follows:
rvm Keeps everything separated from the system in ~/.rvm
version 0.0.25 coming out in a day or so will have support for:
* Named releases, named svn tags, svn revisions, svn head for 1.[8-9].X
* Rubinius, jRuby, REE
* Named Gem Sets (eg, being able to test different sets of gems against a single version of ruby)
* And much, much more... Hop on in #rvm for more or visit http://rvm.beginrescueend.com/ once 0.0.25 is out.