You can also install gems in your local environment (without sudo) with
gem install --user-install <gemname>
I recommend that so you don't mess with your system-level configuration even if it's a single-user computer.
You can check where the gems go by looking at gempaths with gem environment. In my case it's "~/.gem/ruby/1.8".
If you need some binaries from local installs added to your path, you can add something to your bashrc like:
if which ruby >/dev/null && which gem >/dev/null; then
PATH="$(ruby -r rubygems -e 'puts Gem.user_dir')/bin:$PATH"
fi