I have written a similar search to the one you describe, this is the approach I took.

1. build an indexer which takes all the words used in the search and put them into a table with these columns.

word, record_id, word_count, word_in_title

word == the word itself
record_id == where the word points to
word_count == frequency of word in document
word_in_title == does word exist in title

searching for terms than goes like this( notice that I only put the % at the end of the like so it will still use an index! )

select record_id, sum(word_count), max(word_in_title) from TABLE where word like '$word%' group by record_id

if the user searches for multiple words than run the above statement multiple times and sum them up by record_id.

Spit them back out by the max word_count.

My example site: www.historychannel.com ( does no caching of results, still very quick )