Have a look through the PayPal configuration guide on this page:
http://nbstore.codeplex.com/releases/view/34143
Personally I never rely on the sandbox because I can never be sure it is operating correctly... and then you still have to do it live anyway before going live.
Try your store with live PayPal settings and if it takes you through to the PayPal payment page, then you know it is likely to work. Before you go live, set up a one dollar product and make a live purchase as a final test.
Rob