Actually the versions of Ruby shouldn't cause too many problems at all. I'm using 1.8.7 on my servers, 1.8.6 on my work machine (because getting the rights to upgrade is too much of a hassle) and I've been trying out 1.9 on my home machine for a few months. I've only had two issues with 1.9, both to do with old third party plugins and not Rails.
Rails has changed quite a bit over the years. A tutorial for 1.2 will be almost entirely different than a tutorial for 2.3, so its a good idea to make sure those match. You can install multiple version of Rails so it's not much of an issue.