Capture user data before/after Paypal payment

I’m currently working on a site that sells a subscription to a hard-copy magazine. What I need to do is capture the user’s details and store it in the database either before or after they have made a payment to Paypal - I’m guessing that after is probably better in case they don’t complete the transaction. Any guidance or suggestions on how to achieve this would be very much appreciated.

It seems that PayPal’s IPN is the way forward.