ATMEGA chip compatability and FSBUS

Good Morning/evening all.I'm doing too much reading here and getting myself confused.The MegaJoy uses the ATMEGA16-16PI.The FSBUS uses the ATMEGA8535-16PI.Q1)Are these basically the same chips and interchangeable?Q2)Can a ATMEGA8353-16PU be substituted for the ATMEGA8353-16PI?Does anyone know of the best (includes cheapest) supplier of these chips in Australia. I'm running out and can't remember where I got mine.Many thanks in advance.Rob.Melbourne Australia.