Toggle CCK field visibility with CCK radio buttons
There is a contrib module written for just this: Conditional Fields. Here is a quote about it (from the module's project page):

... allows you to manage sets of dependencies between fields. When a field is “dependent”, it will only be available for editing and displayed if the state of the “dependee” field matches the right condition.
When editing a node (or any other entity type that supports fields, like users and categories), the dependent fields are dynamically modified with the States API.
You can, for example, define a custom “Article teaser&quot; field that is shown only if a &quot;Has teaser&quot; checkbox is checked.