After installing Ruby and Ruby Gems:
$ sudo apt-get install ruby rubygems
...
$ ruby -v
ruby 1.8.7 (2010-06-23 patchlevel 299) [i686-linux]
$ gem -v
1.3.7
>
I very strongly suggest you skip the Ubuntu packages and use RVM. The install process is pretty straight forward and documented. There is also a Railscast on it.
RVM will allow you to install multiple versions and create custom gemsets (no dependency problems). It is WELL worth it and quickly becoming the de-facto way to develop (and deploy) with Ruby.