Thanks for the recommendations, they were very helpful.  (Note- Bob, 
I found no setup files on your site but the diagram was very helpful; 
Erik, your diagram was really helpful and made it simple to see what 
went where in the configuration files, something that isn;t always 
easy with NetBSD).

It looks like most of the configuration files are not radically 
different from what was set up to allow my NetBSD machine to use 
Ethernet and my DSL modem to access the Internet.  The changes seem 
to be:

/etc/rc.conf
defaultroute="my isp's gateway" (currently set to my LAN's gateway)
ipfilter=YES


/etc/ipnat.conf
map sn0 192.168.208.0/24 -> my-IP-address portmap tcp/udp 10000:40000

/etc/hosts
#This machine
my-IP-address  sugaree  sugaree.onvoy.com
#Some local machines
192.168.208.1  tims-imac  tims-imac.onvoy.com
192.168.208.3  pb145b  pb145b.onvoy.com

I do also have an /etc/ifconfig.sn0 for the AAUI Ethernet port on the 
Q700/NetBSD machine.

In the past, I have had both the iMac and the NetBSD machine 
configured to use my static IP address and I'd just physically 
alternate which was plugged into the DSL router/hub.  I switched to 
using IPNetRouter to see if it was possible to use both computers 
simultaneously through the DSL and of course it was.  IPNR is beta 
and not especially stable, not to mention $89, so it makes sense to 
use the capabilities of NetBSD in this capacity since it's stable and 
free.

I'm using 192.168.208.x IP addresses for my LAN simply because that's 
what IPNetRouter (IPNR) generated when it was configured, and it 
works with those numbers.  The Q700/NetBSD machine is assigned 
192.168.208.2 currently.  I don't see anywhere to specify this in the 
configuration files, but perhaps I don't need to since this will be 
the exposed computer on the Internet and will be using the IP address 
assigned by my ISP.  Currently the exposed computer is the iMac 
running IPNR.

Now, do I specify the NetBSD as the gateway for my LAN using my 
static IP address or using the fake IP address 192.168.208.2 (I'm 
referring to setting up Open Transport since the other computers will 
be running  under  Mac OS... for now)?

I have one other issue, which is that I am currently serving my 
employer's Web site from the iMac.  How do I configure NAT so that 
requests for Web pages (the domain name is pointed at my IP address) 
get passed on to the iMac?  Eventually I plan on serving this from 
the NetBSD machine using apache, which also makes it sensible to have 
the NetBSD machine be the exposed host on the Internet, but one thing 
at a time.  ;-)

So, do these look like this will work?

Thanks so much!

Tim