Gmail Lab announced a new feature especially useful for lazy people. This is called Canned Responses, or email for the truly lazy.
If you're sick of typing out the same reply every time someone emails you with a common question, now you can compose your reply once and save the message text with the "Canned responses" button. Later, you can open that same message and send it again and again.
Gmail already lets you create filters based on a combination of keywords, sender, recipients, and more in your incoming messages.
Turn on Canned Responses in Labs, and you can set a filter to grab one of your saved responses, create an automated reply, and hit the Send button for you.
You can set up different automated messages for different keywords, just like you said you wanted.