I have a Quick Silver 933 Power Mac from 2002 and it runs Leopard fine. That is an 8 year old Computer. The only issue I have is Flash video or Streaming is almost none playable. However, any h.264 videos that are downloaded run fine. This was our first Mac I bought used to see if Switching made sense for us. I had the Mac for a few hours and decided never to go back to Windows or even Linux. We have since purchased 2 Intel based Macs and the Power Mac is now our kids computer.
I listen to the 'e' format audible boks on my iPhone without any issues. I would assume that the iPad would also have no issues. There was no firmware update required when Audible added the 'e' format. I just changed the setting in my audible preference to download the new format and it just worked.