Efficient Searching of Large Text Fields

In another post on a different topic, Rod Taylor said the following:

"A\tcat in the\nhat" might be stored as ARRAY['A', 'cat', 'in', 'the', 'hat'].

This got me thinking. I have a discussion forum for gamers and want
to provide searching capabilities so the user can type in a phrase
like "magical bow" and get all posts, sorted by relevance that contain
these words.

My questions are:
1. Will storing the posts in an ARRAY help improve performance of
these searches? If so, by how much?
2. What functions or libraries are available to make such searching
easy to implement well?
3. What is the best way to sort by relevance?

Thanks,
Aaron Bono