To answer my own question of rubygems support under EL5, I see
is the keeper of rubygems, and earlier versions can be retrieved
there. From History.txt in the tarball, I see reference that at
rubygems 1.0.1, support for Ruby 1.8.3 - 1.8.5 is fixed, and is
possibly the latest version of rubygems to do so?
For EL4, which runs Ruby 1.8.1, I see that
others have gone
back to source, specifically with Ruby 1.8.4 and Rubygems
0.8.11 giving a version mix that worked for them at the time.