NIC appears in lspci but not in ifconfig
To make sure the card was recognized and a driver loaded, get a shell (in a terminal or remotely) and type the command
dmesg | less
to browse through the kernel messages at boot time. In the less browser, type /eth<enter> to find and highlight the string "eth" and q to quit back to the shell.
There's a message when the driver for the card is loaded.
The Ubuntu desktop has a little network icon, two PCs next to each other. By default it's in the top tray towards the right. There's a little animation when it connects or disconnects.
Next type the commands
ip addr show
and you should see the at least one "eth" device. Two if the 3com driver has loaded.
and you should see the route the DHCP client got via the Intel Ethernet.
If the card is recognized, you can put away the shell window for now and establish a network interface on it with Ubuntu's nice little graphical user interface for that.
Or you can use the shell. Get another shell window and run the command
tail -F /var/log/messages
and just let it run where you can see it. If the driver hasn't loaded yet, the next command will bring it out and you should see a message there. If stuff goes wrong, that's the message we want to see here.
Let's say your home LAN will be 192.168.9.0-255 and the new 3com NIC is eth1. Type this
sudo ifconfig eth1 broadcast 192.168.9.255 netmask 255.255.255.0 192.168.9.1
and then do this again to see if its output changes.
ip addr show ; /sbin/route -n
There should now be an address and a route for eth1 showing.
Now you can go back to Ubuntu's graphical controls and use Synaptic to get the dhcp3-server package and set that up.