John:

It's not an ISO image as you requested, but you can upgrade the AS 
200's firmware using the floppy disk or network bootable images 
located at

   http://ftp.digital.com/pub/DEC/Alpha/firmware/archive/astn200.html

Sorry, don't know the answers to your DE500 questions--I've only used 
the built-in NIC and a 3Com 3c905C-TX in mine.


Ray