Problems with RJS & Ajax

I try to create an "onKeypress" event handler for a form but nothing
works!

I have already studied the reference books on Ajax, but that didn't
help.
Can someone explain me how todo this?

These are the code-snippets:

1) application.js

var GridControlTracker = {}
GridControlTracker = {
  gridEventHandler: function(element, value) {
    alert('Key Pressed / element: '+ element.id);
  }
}

2) show.rhtml

<%= observe_form "grid-form",
                 :frequency => 0,
                 :function =>
"GridControlTracker.gridEventHandler(element,value)",
                 :on => "keypress" %>

this generates this javascript in the page:

<script type="text/javascript">
//<![CDATA[
new Form.Observer('grid-form', 0, function(element, value)
{GridControlTracker.gridEventHandler(element,value)}, 'keypress')
//]]>
</script>

These are the questions that I have:
1. Only the <backspace> and new input-characters are triggered.
    But I would like to trigger the navigation buttons (left, right,
up, down, home ...)
2. I can't find the way to work with the event object. It must be
instantiated, but how can I work with it?
3. I don't see the usage of observe_form with the function option
(instead of URL) in code examples. Is this the correct way? I want
to filter the events that I want to handle and than go to the server
for the selected events. Is this the only way to do this?

thanks and sjamayee!