I am not familiar with this piece of Hardware, but I will give it a shut, so take it with a grain of salt.
The Cisco 678 appears to be combination of DSL modem + one port Ethernet Router with DHCP, and PAT capacity.
DHCP means that you don?t have to assign IP numbers to your computers. The Router will take care of it.
PAT means that you don?t need ICS. The PAT (NAT) will control the Internet distribution.
Since this is a ONE port, Router and you have more then one Computer you will have to add a Switch to add few ports.
Your Connection is:
Phone-DSL-(Cisco 678)-Switch- Computers
Each computer is pluged to a Switch Port (order does not matter).
If your switch has an Uplink connector you connect the Cisco 678 with straight CAT5 cable, otherwise use a Crossover cable to a regular port on the switch.
Set up should be like any Cable/DSL Router. Go back to my Post above.
As for setting the Internet follow your ISP instructions, I have no clue how your ISP is setting the Connections.