An ethernet card is as easy to install as any other PCI card. Turn off, put it in, turn on. YaST should detect and configure it for you. Most routers support DHCP for automatic configuration, so plug it in and it will be set up for you. That should be all you need to do if you have already configured the router with your login details from Windows. To do any other configuration, use your browser to connect to the address given in the router's manual.
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)