ChrisCodes.com

you'll need ruby, rdoc, etc. already installed
# apt-get install ruby rdoc

or, if that gives you problems
# apt-get install ruby1.8 rdoc1.8

debian package for 0.8.4 is here
http://people.debian.org/~akira/rubygems/
download, and
# dpkg -i rubygems_0.8.4-0.1_all.deb

Then update rubygems
# gem install rubygems-update# update_rubygems

proceed with rails install
# gem install rails