been there,
Turn off all PCs and unplug your modem.
place the utp/rj45 connector from your cable modem into the uplink of the 4banger(ensure the button is in) place the PCs into any of the other three ports. Plug in the modem and when the lightsstop flashing, turn on your computers. Make sure the PC with the internet connection in the TCP/IP configurations has connectivity. This is the first fix for the problem in your network topology. And it will allow you to take out that additional nicand free up some resources. Now call your Cable-ISP and add an adition IP address to your account. (5 bucks) and specify the settings in your newly activated/networked machine.
If you realy want to keep the two nic setup, then plug the modem into the nic w/ the correct IP address for your ISP. Then connect a Cross-over cable from the second nic directly to the other PC. This will allow you to share your internet connection without paying the five bucks. Only now you are going to need a degree in network engeneering to config and subnet your 2 computers, and you are going to Split your bandwidth when both machines are connected.
This is why you should pay 5 bucks and get another IP address assigned to your machine and have the super-speed you wanted in the first place.
Oh by the way, I have that NT Degree and I still pay for the second IP... because of the speed issue.