you dont need 2 network cards in 1 computer. but no they cant tell.
the router acts as a hub. i have my 5 port wireless router here. just plug your cable modem into the router onto the end. then each computer into each slot. usally the router uses the web usally
http://192.168.0.1 to configure it
it feeds out a private ip for each computer on the local network then 1 external ip which your isp sees.