Regarding Capybara's Ruby version dependency, I went to the capybara source code and read its History. Searching for "Ruby" quickly got me to the statement that Capybara dropped support for Ruby 1.8 in version 2.0.0. So the previous version, 1.1.4, is the most recent version compatible with Ruby 1.8.
Unfortunately that file says nothing about Rails versions. My Rails 2 projects used webrat, so I don't have any personal data points. However, Googling '"rails 2" capybara version' turns up examples of using Capybara 1.1 with Rails 2 (for example in the Cucumber documentation), so the most recent Capybara version that is compatible with your Ruby is also compatible with your Rails.