Extracting an array of keywords from a post...

Anybody know of a method that will find keywords in a post quickly, and intelligently?

I'm working on a form right now that will be used for people to submit messages, comments, questions, etc. When they submit a comment, rather than send the email to me, I want to first extract keywords from their text, search those against a database of articles, suggest the articles to them, and then submit the question only after they click "None of these help". So basically, I'm forcing them to use the resources already in place before wasting my time.

Any suggestions on how I should find the REAL keywords in these posts? Obviously I'll strip out 'a', 'and', 'of', etc...but I'm still a little in the dark on the full operation.

Thanks in advance.