Hi there,
What other devices have you got on your network ?
What type of router modem have you got ?
What IP address is your router ?
(I don't need to know this but you do ;-))
Plug an ethernet cable into the iMac, plug the other end into the router/modem.
Then as you have done go to the TCP/IP Control panel,set using ethernet and select DHCP, then set the name server as the 'Router' IP address.
That should be it...
regards
Ric