Adding autocomplete to a CCK text field
Why don't you try Autocomplete Widgets for Text and Number Fields

This module adds 3 autocomplete widgets for CCK fields of type Text and Number.

Autocomplete for allowed values list: This widget can be used for Text and Number fields and it takes candidate values from the defined list of Allowed values of the fields. You can even generate your allowed values list using PHP, so the limit to provide a widget with autocomplete features is your imagination!
Autocomplete for existing field data: This widget can be used for Text only and it takes candidate values from existing values in the database for that field.
(7.x only) Autocomplete for predefined suggestions: This widget can be used for Text only and allows an admin to provide a list of suggestions but still allows users to enter anything they want in a text field. Use this widget to help avoid (but not prevent) variations of the same value. Ex: burger, hamburger, Burger ...
(7.x only) Autocomplete for existing field data and some node titles: This widget works just like the "existing field data" widget above except it will also suggest node titles for nodes of a specific content type(s).