Programmatically download last module version
It's dangerous process what you're looking for.

What if the user has a older CCK version ?
What if the user patched is CCK ?
What if the latest version of CCK won't works ?

Also, it's not fair to download modules without letting knowing the user what you're doing. From my point of view, the best solution it's to add dependency in the info file and with the hook_init you can check the version of CCK and display a message to as the user download the latest version of CCK.