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)

Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&




The newer nics have data processors on them. Im using an ald 10 Mb NIC. The nic itself is new but the model is about 4 years old and I am getting steady 500KB/s dl rate on any online servers that go that high. I am using the driver provided by WinUpdate though. 8) but yeah don't need to waste ure cash on one onboard is perfect.