I see its been awhile since this post...did you ever figure anything out? What you want to accomplish is definitely doable...but I'd be careful using ajax with an onKeyUp event...that means that every time the user presses a key you're going to hit your server again...that is probably going to be excessive. What I would recommend is using
Prototype.js. Take a look at the documentation on that web site. I'd check out Ajax.PeriodicalUpdater
here. That would probably work better than going after an onKeyUp event.
Good Luck!