Visit these awesome links that I found very impressive andknowledgeable.
http://tech-brains.blogspot.in/2012/09/paypal-integration-with-ror.html
http://tech-brains.blogspot.in/2012/09/online-payment-solution-for-ror.html
http://tech-brains.blogspot.in/2012/09/online-payment-solution-for-ror_19.html
Thanks