Dynamic Select List

I have a field called field_server_type that contains the following static items in a select list: AIX, Linux, Windows, Other. I then have another field called field_server_operator. I want to dynamically populate the field_server_operator dynamically based on what the user select in the field_server_type. So if the user selects AIX from the select list in field_server_type, the allowed options would be "Brian" and "Bill" in the field_server_operator. If the user selects other from the field_server_type then it would populate the field_server_operator dynamically with the "Rob".

My guess is that I am using the wrong api call to try and get the information dynamically from the user. I don't think $form_state does what I think it does.

I am sure that there is some fairly easy way to do this. I am a noob when it comes to drupal. Any help you be appreciated. I have included some code I have come up with that does not work but it may better illustrate what I am trying to do. Thank you in advance for any help that you can give.

--Brian

$infra = array("Brian", "Bill");
$net = array("Rob");
$test = (string) $form_state['values']['field_server_type'] ;
if ($test == "AIX"){
    return $infra;
}else{
    return $net;
}