is your onboard NIC gigabit ethernet

If so, it might actually be 2 chips. The MAC portion is integrated in the chipset, such as NVidia 630i, 680i. The PHY portion is in a seperate chip, could be Marvell or Intel. You will have to install the PHY driver in addition to the chipset driver.

The best way to determine is to take a look at the mobe and see what chip is soldered close to the Ethernet port. There are only a handful companies out there who can make gigabit ethernet PHY -- Marvell, Broadcom, LSI/Agere, Intel. Once you know the chip, go to manufacturer's website to download the driver.