A LAN card is just a LAN card, your better off using the onboard ones as they are newer technology as far as performance goes, but the oldest spec LAN card will determine the connection speed.
I think it's nuts to add a LAN card to a mainboard that already has ( two ? ) onboard LAN devices.
The LAN card you got for a bargain price isn't worth two knobs of goat poo if you don't have drivers for it.
Configuring a lan device with (is it three lan cards in your system now? ) multiple cards in the system is gonna be a nightmare for you.
If your mainboard has onboard LAN, use it! if it has two
onboard lan devices, disable one of them, perhaps the one that's newer technology than the card in the second PC. In other words, if the other PC has a 10/100 card, use the onboard equivalent of the 10/100 on your PC.
Someone else might know the details, I'm not exactly familiar with your mainboard's lan technologies, (I'll be finding out for myself soon enough, broadband connection is happening in the next couple weeks)