That's a very good question as the rubygems embedded in the debian package is a limited edition where you are not able to make an "update --system".

So first thing to do:

 sudo apt-get remove rubygems

Then, from www.rubyonrails.com, follow the rubygems installation tutorial:
- download the archive delivered from rubyforge,
- tar -zxvf rubygems-1.3.1.tgz

and:
 cd /rubygems-1.3.1
 sudo ruby setup.rb

and follow with a :
 sudo gem update --system

OK, rubygems is now up to date.

Go on installation of rmagick:

 sudo apt-get install ruby1.8-dev
 sudo apt-get install g++ automake autoconf
 sudo apt-get install libmagick++10
 sudo apt-get install libmagick++9-dev
 sudo gem install rmagick

and then you will get the following console output:

frederic@nostromo2:~/Logiciels/development/ruby/rubygems-1.3.1$ sudo gem install rmagick
Building native extensions.  This could take a while...
Successfully installed rmagick-2.9.0
1 gem installed

And now, you will be able to use the Rails plugin named file_column; cd into your rails project:
 cd myprojectsrails
 script/plugin install http://opensvn.csie.org/rails_file_column/plugins/file_column/trunk

And enjoy !