I just thought I would provide some closure on my 3c509b saga.

I think that if that had been an initial install, current software
would have successfully handled the card.  Apparently the new 3c509
module can cope with the card in PnP mode.  There were two things
I had to do.

I examined my isapnp.conf and saw that the SoundBlaster card was
using the 3c509's default io ports.  Looking at output from pnpdump
and comparing it with isapnp.conf showed what the parameters were.
I also checked the Windows Device Manager Hardware Profiles and
Resources, where I found some alternatives to the current Linux
settings.  I was able to change isapnp.conf to use the alternative
addresses.

The second change I made was to use linuxconf to set the interrupt
and ioport for the 3c509 module, which was added to modules.conf.
(I did notice that despite what was in modules.conf the contents
of /proc/ioports did not match what was in modules.conf.)

The net result is that I can now do "insmod 3c509" and "ifup eth0"
and get my Linux box to talk to the DSL modem and out to the internet.

Thank you all for your advice.

dsc