Assign one role to a user

Hello Guys,

I am planning to allow some users to add users to the site.
The only concern is I want them to select only one user role, so, instead
of a checkboxes, i want a select list or radio buttons.
It can be achieved via form alter, but just wanted to know if there is any
module available for this
I am on Drupal 6.

-- 
Regards,
Vaibhav Jain