Hi Den

The easiest way to do this is by using a module from ucanuse (www.ucanuse.com) I use it extensively on my site, similiar to what you are looking to do. The module is called User Attributes and it  works as expected (the support is excellent too). You can create as many registration scenarios as you want and you also have the ability to add to roles based on what was entered on the registration form etc.

Hope that helps