Which method to set up pay to register? I am building a site that requires payment, from paypal for instance, as part of the user registration. What approach should I take?
Does it have to be registration? You might find it easier to set up one or more roles with the privileges you want to grant to paying users and then use Ubercart to grant these roles to users who buy the appropriate products.

If this sounds good, look at using the uc_roles module that comes with Ubercart. The Ubercart documentation contains a pretty good overview: on selling site access with role assignment.