Don't add another router

If one of your concentrators is a router, when you outgrow the number of ports, don't buy another router; instead, buy a switch. Use one of the router's ports to connect the switch using Ethernet cable. Remember that one of the router's ports is dedicated to the connection to your broadband modem, so don't touch the cable in that port.

If your computers aren't widely separated and the fact that you've outgrown your concentrator is the result of adding more computers than the router has ports, buy a large switch (such as an 8-port or 16-port switch). Connect all of the computers to the switch and then connect the switch to the router. You can use the extra ports on the switch in the future to add more computers, or to add one of those nifty Network Storage Drives (very large hard drives that connect directly to a switch as if they were another computer on the network). These are handy devices for storing backups or media files.