Customizing user registration

I need to customize the registration of a new user.
I read some instructions here:
http://drupal.org/node/154113

However, altering the whole login page won't help me. Instead I was wondering could I make a custom page and then use drupal_execute to submit the build-in user registration form programmatically.

How can I find out what is the form_id of the user registration form?
How can I find out what are the field values of the user registration form?