Membership creation and Paypal

I want to create a new user in the following method

Create user wizard control,
On clicking continue, I want them to forward on to paypal by putting some sort of code behind on the button for the paypal button.
I want to also take the user name and enter that in another database along with the date and time they registered.
Once they hit paypal they pay and it triggers an IPN that will update the 2nd database with a renewal date that is 12 months on from the payment going through.

I have an IPn of sorts but I am not sure how to firstly code the continue button to put the username in a session and also the @@identity into a session that I can use with paypal.
second to then get the ipn to update the database table.

Can anyone assist?

Thanks